|
Network Working Group Request for Comments: 2178 Obsoletes: 1583 Category: Standards Track |
J. Moy Cascade Communications Corp. July 1997 |
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
This memo documents version 2 of the OSPF protocol. OSPF is a link- state routing protocol. It is designed to be run internal to a single Autonomous System. Each OSPF router maintains an identical database describing the Autonomous System's topology. From this database, a routing table is calculated by constructing a shortest- path tree.
OSPF recalculates routes quickly in the face of topological changes, utilizing a minimum of routing protocol traffic. OSPF provides support for equal-cost multipath. An area routing capability is provided, enabling an additional level of routing protection and a reduction in routing protocol traffic. In addition, all OSPF routing protocol exchanges are authenticated.
The differences between this memo and RFC 1583 are explained in Appendix G. All differences are backward-compatible in nature. Implementations of this memo and of RFC 1583 will interoperate.
Please send comments to ospf@gated.cornell.edu.
1 Introduction
1.1 Protocol Overview
1.2 Definitions of commonly used terms
1.3 Brief history of link-state routing technology
1.4 Organization of this document
1.5 Acknowledgments
2 The link-state database: organization and calculations 11
2.1 Representation of routers and networks
2.1.1 Representation of non-broadcast networks
2.1.2 An example link-state database
2.2 The shortest-path tree
2.3 Use of external routing information
2.4 Equal-cost multipath
3 Splitting the AS into Areas
3.1 The backbone of the Autonomous System
3.2 Inter-area routing
3.3 Classification of routers
3.4 A sample area configuration
3.5 IP subnetting support
3.6 Supporting stub areas
3.7 Partitions of areas
4 Functional Summary
4.1 Inter-area routing
4.2 AS external routes
4.3 Routing protocol packets
4.4 Basic implementation requirements
4.5 Optional OSPF capabilities
5 Protocol data structures
6 The Area Data Structure
7 Bringing Up Adjacencies
7.1 The Hello Protocol
7.2 The Synchronization of Databases
7.3 The Designated Router
7.4 The Backup Designated Router
7.5 The graph of adjacencies
8 Protocol Packet Processing
8.1 Sending protocol packets
8.2 Receiving protocol packets
9 The Interface Data Structure
9.1 Interface states
9.2 Events causing interface state changes
9.3 The Interface state machine
9.4 Electing the Designated Router
9.5 Sending Hello packets
9.5.1 Sending Hello packets on NBMA networks
10 The Neighbor Data Structure
10.1 Neighbor states
10.2 Events causing neighbor state changes
10.3 The Neighbor state machine
10.4 Whether tocome adjacent
10.5 Receiving Hello Packets
10.6 Receiving Database Description Packets
10.7 Receiving Link State Request Packets
10.8 Sending Database Description Packets
10.9 Sending Link State Request Packets
10.10 An Example
11 The Routing Table Structure
11.1 Routing table lookup
11.2 Sample routing table, without areas
11.3 Sample routing table, with areas
12 Link State Advertisements (LSAs)
12.1 The LSA Header
12.1.1 LS age
12.1.2 Options
12.1.3 LS type
12.1.4 Link State ID
12.1.5 Advertising Router
12.1.6 LS sequence number
12.1.7 LS checksum
12.2 The link state database
12.3 Representation of TOS
12.4 Originating LSAs
12.4.1 Router-LSAs
12.4.1.1 Describing point-to-point interfaces
12.4.1.2 Describing broadcast and NBMA interfaces
12.4.1.3 Describing virtual links
12.4.1.4 Describing Point-to-MultiPoint interfaces
12.4.1.5 Examples of router-LSAs
12.4.2 Network-LSAs
12.4.2.1 Examples of network-LSAs
12.4.3 Summary-LSAs
12.4.3.1 Originating summary-LSAs into stub areas
12.4.3.2 Examples of summary-LSAs
12.4.4 AS-external-LSAs
12.4.4.1 Examples of AS-external-LSAs
13 The Flooding Procedure
13.1 Determining which LSA is newer
13.2 Installing LSAs in the database
13.3 Next step in the flooding procedure
13.4 Receiving self-originated LSAs
13.5 Sending Link State Acknowledgment packets
13.6 Retransmitting LSAs
13.7 Receiving link state acknowledgments
14 Aging The Link State Database
14.1 Premature aging of LSAs
15 Virtual Links
16 Calculation of the routing table
16.1 Calculating the shortest-path tree for an area
16.1.1 The next hop calculation
16.2 Calculating the inter-area routes
16.3 Examining transit areas' summary-LSAs
16.4 Calculating AS external routes
16.4.1 External path preferences
16.5 Incremental updates -- summary-LSAs
16.6 Incremental updates -- AS-external-LSAs
16.7 Events generated as a result of routing table changes 153
16.8 Equal-cost multipath
G.4 A modification to the flooding algorithm
G.5 Introduction of the MinLSArrival constant
G.6 Optionally advertising point-to-point links as subnets 207
G.7 Advertising same external route from multiple areas .. 207
G.8 Retransmission of initial Database Description packets 209
G.9 Detecting interface MTU mismatches
This document is a specification of the Open Shortest Path First (OSPF) TCP/IP internet routing protocol. OSPF is classified as an Interior Gateway Protocol (IGP). This means that it distributes routing information between routers belonging to a single Autonomous System. The OSPF protocol is based on link-state or SPF technology. This is a departure from the Bellman-Ford base used by traditional TCP/IP internet routing protocols.
The OSPF protocol was developed by the OSPF working group of the Internet Engineering Task Force. It has been designed expressly for the TCP/IP internet environment, including explicit support for CIDR and the tagging of externally-derived routing information. OSPF also provides for the authentication of routing updates, and utilizes IP multicast when sending/receiving the updates. In addition, much work has been done to produce a protocol that responds quickly to topology changes, yet involves small amounts of routing protocol traffic.
OSPF routes IP packets based solely on the destination IP address found in the IP packet header. IP packets are routed "as is" -- they are not encapsulated in any further protocol headers as they transit the Autonomous System. OSPF is a dynamic routing protocol. It quickly detects topological changes in the AS (such as router interface failures) and calculates new loop-free routes after a period of convergence. This period of convergence is short and involves a minimum of routing traffic.
In a link-state routing protocol, each router maintains a database describing the Autonomous System's topology. This database is referred to as the link-state database. Each participating router has an identical database. Each individual piece of this database is a particular router's local state (e.g., the router's usable interfaces and reachable neighbors). The router distributes its local state throughout the Autonomous System by flooding.
All routers run the exact same algorithm, in parallel. From the link-state database, each router constructs a tree of shortest paths with itself as root. This shortest-path tree gives the route to each destination in the Autonomous System. Externally derived routing information appears on the tree as leaves.
When several equal-cost routes to a destination exist, traffic is distributed equally among them. The cost of a route is described by a single dimensionless metric.
OSPF allows sets of networks to be grouped together. Such a grouping is called an area. The topology of an area is hidden from the rest of the Autonomous System. This information hiding enables a significant reduction in routing traffic. Also, routing within the area is determined only by the area's own topology, lending the area protection from bad routing data. An area is a generalization of an IP subnetted network.
OSPF enables the flexible configuration of IP subnets. Each route distributed by OSPF has a destination and mask. Two different subnets of the same IP network number may have different sizes (i.e., different masks). This is commonly referred to as variable length subnetting. A packet is routed to the best (i.e., longest or most specific) match. Host routes are considered to be subnets whose masks are "all ones" (0xffffffff).
All OSPF protocol exchanges are authenticated. This means that only trusted routers can participate in the Autonomous System's routing. A variety of authentication schemes can be used; in fact, separate authentication schemes can be configured for each IP subnet.
Externally derived routing data (e.g., routes learned from an Exterior Gateway Protocol such as BGP; see [Ref23]) is advertised throughout the Autonomous System. This externally derived data is kept separate from the OSPF protocol's link state data. Each external route can also be tagged by the advertising router, enabling the passing of additional information between routers on the boundary of the Autonomous System.
This section provides definitions for terms that have a specific meaning to the OSPF protocol and that are used throughout the text. The reader unfamiliar with the Internet Protocol Suite is referred to [Ref13] for an introduction to IP.
Router
A level three Internet Protocol packet switch. Formerly called a
gateway in much of the IP literature.
Autonomous System
A group of routers exchanging routing information via a common
routing protocol. Abbreviated as AS.
Interior Gateway Protocol
The routing protocol spoken by the routers belonging to an
Autonomous system. Abbreviated as IGP. Each Autonomous System has
a single IGP. Separate Autonomous Systems may be running
different IGPs.
Router ID
A 32-bit number assigned to each router running the OSPF protocol.
This number uniquely identifies the router within an Autonomous
System.
Network
In this memo, an IP network/subnet/supernet. It is possible for
one physical network to be assigned multiple IP network/subnet
numbers. We consider these to be separate networks. Point-to-
point physical networks are an exception - they are considered a
single network no matter how many (if any at all) IP
network/subnet numbers are assigned to them.
Network mask
A 32-bit number indicating the range of IP addresses residing on a
single IP network/subnet/supernet. This specification displays
network masks as hexadecimal numbers. For example, the network
mask for a class C IP network is displayed as 0xffffff00. Such a
mask is often displayed elsewhere in the literature as
255.255.255.0.
Point-to-point networks
A network that joins a single pair of routers. A 56Kb serial line
is an example of a point-to-point network.
Broadcast networks
Networks supporting many (more than two) attached routers,
together with the capability to address a single physical message
to all of the attached routers (broadcast). Neighboring routers
are discovered dynamically on these nets using OSPF's Hello
Protocol. The Hello Protocol itself takes advantage of the
broadcast capability. The OSPF protocol makes further use of
multicast capabilities, if they exist. Each pair of routers on a
broadcast network is assumed to be able to communicate directly.
An ethernet is an example of a broadcast network.
Non-broadcast networks
Networks supporting many (more than two) routers, but having no
broadcast capability. Neighboring routers are maintained on these
nets using OSPF's Hello Protocol. However, due to the lack of
broadcast capability, some configuration information may be
necessary to aid in the discovery of neighbors. On non-broadcast
networks, OSPF protocol packets that are normally multicast need
to be sent to each neighboring router, in turn. An X.25 Public
Data Network (PDN) is an example of a non-broadcast network.
OSPF runs in one of two modes over non-broadcast networks. The first mode, called non-broadcast multi-access or NBMA, simulates the operation of OSPF on a broadcast network. The second mode, called Point-to-MultiPoint, treats the non-broadcast network as a collection of point-to-point links. Non-broadcast networks are referred to as NBMA networks or Point-to-MultiPoint networks, depending on OSPF's mode of operation over the network.
Interface
The connection between a router and one of its attached networks.
An interface has state information associated with it, which is
obtained from the underlying lower level protocols and the routing
protocol itself. An interface to a network has associated with it
a single IP address and mask (unless the network is an unnumbered
point-to-point network). An interface is sometimes also referred
to as a link.
Neighboring routers
Two routers that have interfaces to a common network. Neighbor
relationships are maintained by, and usually dynamically
discovered by, OSPF's Hello Protocol.
Adjacency
A relationship formed between selected neighboring routers for the
purpose of exchanging routing information. Not every pair of
neighboring routers become adjacent.
Link state advertisement
Unit of data describing the local state of a router or network.
For a router, this includes the state of the router's interfaces
and adjacencies. Each link state advertisement is flooded
throughout the routing domain. The collected link state
advertisements of all routers and networks forms the protocol's
link state database. Throughout this memo, link state
advertisement is abbreviated as LSA.
Hello Protocol
The part of the OSPF protocol used to establish and maintain
neighbor relationships. On broadcast networks the Hello Protocol
can also dynamically discover neighboring routers.
Flooding
The part of the OSPF protocol that distributes and synchronizes
the link-state database between OSPF routers.
Designated Router
Each broadcast and NBMA network that has at least two attached
routers has a Designated Router. The Designated Router generates
an LSA for the network and has other special responsibilities in
the running of the protocol. The Designated Router is elected by
the Hello Protocol.
The Designated Router concept enables a reduction in the number of adjacencies required on a broadcast or NBMA network. This in turn reduces the amount of routing protocol traffic and the size of the link-state database.
Lower-level protocols
The underlying network access protocols that provide services to
the Internet Protocol and in turn the OSPF protocol. Examples of
these are the X.25 packet and frame levels for X.25 PDNs, and the
ethernet data link layer for ethernets.
OSPF is a link state routing protocol. Such protocols are also referred to in the literature as SPF-based or distributed-database protocols. This section gives a brief description of the developments in link-state technology that have influenced the OSPF protocol.
The first link-state routing protocol was developed for use in the ARPANET packet switching network. This protocol is described in [Ref3]. It has formed the starting point for all other link-state protocols. The homogeneous ARPANET environment, i.e., single-vendor
packet switches connected by synchronous serial lines, simplified the design and implementation of the original protocol.
Modifications to this protocol were proposed in [Ref4]. These modifications dealt with increasing the fault tolerance of the routing protocol through, among other things, adding a checksum to the LSAs (thereby detecting database corruption). The paper also included means for reducing the routing traffic overhead in a link- state protocol. This was accomplished by introducing mechanisms which enabled the interval between LSA originations to be increased by an order of magnitude.
A link-state algorithm has also been proposed for use as an ISO IS-IS routing protocol. This protocol is described in [Ref2]. The protocol includes methods for data and routing traffic reduction when operating over broadcast networks. This is accomplished by election of a Designated Router for each broadcast network, which then originates an LSA for the network.
The OSPF Working Group of the IETF has extended this work in developing the OSPF protocol. The Designated Router concept has been greatly enhanced to further reduce the amount of routing traffic required. Multicast capabilities are utilized for additional routing bandwidth reduction. An area routing scheme has been developed enabling information hiding/protection/reduction. Finally, the algorithms have been tailored for efficient operation in TCP/IP internets.
The first three sections of this specification give a general overview of the protocol's capabilities and functions. Sections 4-16 explain the protocol's mechanisms in detail. Packet formats, protocol constants and configuration items are specified in the appendices.
Labels such as HelloInterval encountered in the text refer to protocol constants. They may or may not be configurable. Architectural constants are summarized in Appendix B. Configurable constants are summarized in Appendix C.
The detailed specification of the protocol is presented in terms of data structures. This is done in order to make the explanation more precise. Implementations of the protocol are required to support the functionality described, but need not use the precise data structures that appear in this memo.
The author would like to thank Ran Atkinson, Fred Baker, Jeffrey Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra Jujjavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui Zhang and the rest of the OSPF Working Group for the ideas and support they have given to this project.
The OSPF Point-to-MultiPoint interface is based on work done by Fred Baker.
The OSPF Cryptographic Authentication option was developed by Fred Baker and Ran Atkinson.
The following subsections describe the organization of OSPF's link- state database, and the routing calculations that are performed on the database in order to produce a router's routing table.
The Autonomous System's link-state database describes a directed graph. The vertices of the graph consist of routers and networks. A graph edge connects two routers when they are attached via a physical point-to-point network. An edge connecting a router to a network indicates that the router has an interface on the network. Networks can be either transit or stub networks. Transit networks are those capable of carrying data traffic that is neither locally originated nor locally destined. A transit network is represented by a graph vertex having both incoming and outgoing edges. A stub network's vertex has only incoming edges.
The neighborhood of each network node in the graph depends on the network's type (point-to-point, broadcast, NBMA or Point-to- MultiPoint) and the number of routers having an interface to the network. Three cases are depicted in Figure 1a. Rectangles indicate routers. Circles and oblongs indicate networks. Router names are prefixed with the letters RT and network names with the letter N. Router interface names are prefixed by the letter I. Lines between routers indicate point-to-point networks. The left side of the figure shows networks with their connected routers, with the resulting graphs shown on the right.
**FROM**
* |RT1|RT2|
+---+Ia +---+ * ------------
|RT1|------|RT2| T RT1| | X |
+---+ Ib+---+ O RT2| X | |
* Ia| | X |
* Ib| X | |
Physical point-to-point networks
**FROM**
+---+ *
|RT7| * |RT7| N3|
+---+ T ------------
| O RT7| | |
+----------------------+ * N3| X | |
N3 *
Stub networks
+---+ +---+
|RT3| |RT4| |RT3|RT4|RT5|RT6|N2 |
+---+ +---+ * ------------------------
| N2 | * RT3| | | | | X |
+----------------------+ T RT4| | | | | X |
| | O RT5| | | | | X |
+---+ +---+ * RT6| | | | | X |
|RT5| |RT6| * N2| X | X | X | X | |
+---+ +---+
Broadcast or NBMA networks
Figure 1a: Network map components
Networks and routers are represented by vertices. An edge connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X.
The top of Figure 1a shows two routers connected by a point-to-point link. In the resulting link-state database graph, the two router vertices are directly connected by a pair of edges, one in each direction. Interfaces to point-to-point networks need not be assigned IP addresses. When interface addresses are assigned, they are modelled as stub links, with each router advertising a stub connection to the other router's interface address. Optionally, an IP
subnet can be assigned to the point-to-point network. In this case, both routers advertise a stub link to the IP subnet, instead of advertising each others' IP interface addresses.
The middle of Figure 1a shows a network with only one attached router (i.e., a stub network). In this case, the network appears on the end of a stub connection in the link-state database's graph.
When multiple routers are attached to a broadcast network, the link- state database graph shows all routers bidirectionally connected to the network vertex. This is pictured at the bottom of Figure 1a.
Each network (stub or transit) in the graph has an IP address and associated network mask. The mask indicates the number of nodes on the network. Hosts attached directly to routers (referred to as host routes) appear on the graph as stub networks. The network mask for a host route is always 0xffffffff, which indicates the presence of a single node.
As mentioned previously, OSPF can run over non-broadcast networks in one of two modes: NBMA or Point-to-MultiPoint. The choice of mode determines the way that the Hello protocol and flooding work over the non-broadcast network, and the way that the network is represented in the link-state database.
In NBMA mode, OSPF emulates operation over a broadcast network: a Designated Router is elected for the NBMA network, and the Designated Router originates an LSA for the network. The graph representation for broadcast networks and NBMA networks is identical. This representation is pictured in the middle of Figure 1a.
NBMA mode is the most efficient way to run OSPF over non-broadcast networks, both in terms of link-state database size and in terms of the amount of routing protocol traffic. However, it has one significant restriction: it requires all routers attached to the NBMA network to be able to communicate directly. This restriction may be met on some non-broadcast networks, such as an ATM subnet utilizing SVCs. But it is often not met on other non-broadcast networks, such as PVC-only Frame Relay networks. On non-broadcast networks where not all routers can communicate directly you can break the non-broadcast network into logical subnets, with the routers on each subnet being able to communicate directly, and then run each separate subnet as an NBMA network (see [Ref15]). This however requires quite a bit of administrative overhead, and is prone to misconfiguration. It is probably better to run such a non-broadcast network in Point-to- Multipoint mode.
In Point-to-MultiPoint mode, OSPF treats all router-to-router connections over the non-broadcast network as if they were point-to- point links. No Designated Router is elected for the network, nor is there an LSA generated for the network. In fact, a vertex for the Point-to-MultiPoint network does not appear in the graph of the link-state database.
Figure 1b illustrates the link-state database representation of a Point-to-MultiPoint network. On the left side of the figure, a Point-to-MultiPoint network is pictured. It is assumed that all routers can communicate directly, except for routers RT4 and RT5. I3 though I6 indicate the routers' IP interface addresses on the Point- to-MultiPoint network. In the graphical representation of the link- state database, routers that can communicate directly over the Point-to-MultiPoint network are joined by bidirectional edges, and each router also has a stub connection to its own IP interface address (which is in contrast to the representation of real point- to-point links; see Figure 1a).
On some non-broadcast networks, use of Point-to-MultiPoint mode and data-link protocols such as Inverse ARP (see [Ref14]) will allow autodiscovery of OSPF neighbors even though broadcast support is not available.
Figure 2 shows a sample map of an Autonomous System. The rectangle labelled H1 indicates a host, which has a SLIP connection to Router RT12. Router RT12 is therefore advertising a host route. Lines between routers indicate physical point-to-point networks. The only point-to-point network that has been assigned interface addresses is the one joining Routers RT6 and RT10. Routers RT5 and RT7 have BGP connections to other Autonomous Systems. A set of BGP-learned routes have been displayed for both of these routers.
A cost is associated with the output side of each router interface. This cost is configurable by the system administrator. The lower the cost,the more likely the interface is to be used to forward data traffic. Costs are also associated with the externally derived routing data (e.g., the BGP-learned routes).
The directed graph resulting from the map in Figure 2 is depicted in Figure 3. Arcs are labelled with the cost of the corresponding router output interface. Arcs having no labelled cost have a cost of 0. Note that arcs leading from networks to routers always have cost 0; they are significant nonetheless. Note also that the externally derived routing data appears on the graph as stubs.
**FROM**
+---+ +---+
|RT3| |RT4| |RT3|RT4|RT5|RT6|
+---+ +---+ * --------------------
I3| N2 |I4 * RT3| | X | X | X |
+----------------------+ T RT4| X | | | X |
I5| |I6 O RT5| X | | | X |
+---+ +---+ * RT6| X | X | X | |
|RT5| |RT6| * I3| X | | | |
+---+ +---+ I4| | X | | |
I5| | | X | |
I6| | | | X |
Figure 1b: Network map components
Point-to-MultiPoint networks
All routers can communicate directly over N2, except
routers RT4 and RT5. I3 through I6 indicate IP
interface addresses
+
| 3+---+ N12 N14
N1|--|RT1|\ 1 \ N13 /
| +---+ \ 8\ |8/8
+ \ ____ \|/
/ \ 1+---+8 8+---+6
* N3 *---|RT4|------|RT5|--------+
\____/ +---+ +---+ |
+ / | |7 |
| 3+---+ / | | |
N2|--|RT2|/1 |1 |6 |
| +---+ +---+8 6+---+ |
+ |RT3|--------------|RT6| |
+---+ +---+ |
|2 Ia|7 |
| | |
+---------+ | |
N4 | |
| |
| |
N11 | |
+---------+ | |
| | | N12
|3 | |6 2/
+---+ | +---+/
|RT9| | |RT7|---N15
+---+ | +---+ 9
|1 + | |1
_|__ | Ib|5 __|_
/ \ 1+----+2 | 3+----+1 / \
* N9 *------|RT11|----|---|RT10|---* N6 *
\____/ +----+ | +----+ \____/
| | |
|1 + |1
+--+ 10+----+ N8 +---+
|H1|-----|RT12| |RT8|
+--+SLIP +----+ +---+
|2 |4
| |
+---------+ +--------+
N10 N7
Figure 2: A sample Autonomous System
**FROM**
|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
----- ---------------------------------------------
RT1| | | | | | | | | | | | |0 | | | |
RT2| | | | | | | | | | | | |0 | | | |
RT3| | | | | |6 | | | | | | |0 | | | |
RT4| | | | |8 | | | | | | | |0 | | | |
RT5| | | |8 | |6 |6 | | | | | | | | | |
RT6| | |8 | |7 | | | | |5 | | | | | | |
RT7| | | | |6 | | | | | | | | |0 | | |
* RT8| | | | | | | | | | | | | |0 | | |
* RT9| | | | | | | | | | | | | | | |0 |
T RT10| | | | | |7 | | | | | | | |0 |0 | |
O RT11| | | | | | | | | | | | | | |0 |0 |
* RT12| | | | | | | | | | | | | | | |0 |
* N1|3 | | | | | | | | | | | | | | | |
N2| |3 | | | | | | | | | | | | | | |
N3|1 |1 |1 |1 | | | | | | | | | | | | |
N4| | |2 | | | | | | | | | | | | | |
N6| | | | | | |1 |1 | |1 | | | | | | |
N7| | | | | | | |4 | | | | | | | | |
N8| | | | | | | | | |3 |2 | | | | | |
N9| | | | | | | | |1 | |1 |1 | | | | |
N10| | | | | | | | | | | |2 | | | | |
N11| | | | | | | | |3 | | | | | | | |
N12| | | | |8 | |2 | | | | | | | | | |
N13| | | | |8 | | | | | | | | | | | |
N14| | | | |8 | | | | | | | | | | | |
N15| | | | | | |9 | | | | | | | | | |
H1| | | | | | | | | | | |10| | | | |
Figure 3: The resulting directed graph
Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X.
The link-state database is pieced together from LSAs generated by the routers. In the associated graphical representation, the neighborhood of each router or transit network is represented in a single, separate LSA. Figure 4 shows these LSAs graphically. Router RT12 has an interface to two broadcast networks and a SLIP line to a host. Network N6 is a broadcast network with three attached routers. The cost of all links from Network N6 to its attached routers is 0.
Note that the LSA for Network N6 is actually generated by one of the network's attached routers: the router that has been elected Designated Router for the network.
When no OSPF areas are configured, each router in the Autonomous System has an identical link-state database, leading to an identical graphical representation. A router generates its routing table from this graph by calculating a tree of shortest paths with the router itself as root. Obviously, the shortest- path tree depends on the router doing the calculation. The shortest-path tree for Router RT6 in our example is depicted in Figure 5.
The tree gives the entire path to any destination network or host. However, only the next hop to the destination is used in the forwarding process. Note also that the best route to any router has also been calculated. For the processing of external data, we note the next hop and distance to any router advertising external routes. The resulting routing table for Router RT6 is pictured in Table 2. Note that there is a separate route for each end of a numbered point-to-point network (in this case, the serial line between Routers RT6 and RT10).
**FROM** **FROM**
|RT12|N9|N10|H1| |RT9|RT11|RT12|N9|
* -------------------- * ----------------------
* RT12| | | | | * RT9| | | |0 |
T N9|1 | | | | T RT11| | | |0 |
O N10|2 | | | | O RT12| | | |0 |
* H1|10 | | | | * N9| | | | |
* *
RT12's router-LSA N9's network-LSA
Figure 4: Individual link state components
Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X.
RT6(origin)
RT5 o------------o-----------o Ib
/|\ 6 |\ 7
8/8|8\ | \
/ | \ 6| \
N12 o N14 | \
N13 2 | \
N4 o-----o RT3 \
/ \ 5
1/ RT10 o-------o Ia
/ |\
RT4 o-----o N3 3| \1
/| | \ N6 RT7
/ | N8 o o---------o
/ | | | /|
RT2 o o RT1 | | 2/ |9
/ | | |RT8 / |
/3 |3 RT11 o o o o
/ | | | N12 N15
N2 o o N1 1| |4
| |
N9 o o N7
/|
/ |
N11 RT9 / |RT12
o--------o-------o o--------o H1
3 | 10
|2
|
Figure 5: The SPF tree for Router RT6
Edges that are not marked with a cost have a cost of of zero (these are network-to-router links). Routes to networks N12-N15 are external information that is considered in Section 2.3
Destination Next Hop Distance
__________________________________
N1 RT3 10
N2 RT3 10
N3 RT3 7
N4 RT3 8
Ib * 7
Ia RT10 12
N6 RT10 8
N7 RT10 12
N8 RT10 10
N9 RT10 11
N10 RT10 13
N11 RT10 14
H1 RT10 21
__________________________________
RT5 RT5 6
RT7 RT10 8
Table 2: The portion of Router RT6's routing table listing local destinations.
Routes to networks belonging to other AS'es (such as N12) appear as dashed lines on the shortest path tree in Figure 5. Use of this externally derived routing information is considered in the next section.
After the tree is created the external routing information is examined. This external routing information may originate from another routing protocol such as BGP, or be statically configured (static routes). Default routes can also be included as part of the Autonomous System's external routing information.
External routing information is flooded unaltered throughout the AS. In our example, all the routers in the Autonomous System know that Router RT7 has two external routes, with metrics 2 and 9.
OSPF supports two types of external metrics. Type 1 external metrics are expressed in the same units as OSPF interface cost (i.e., in terms of the link state metric). Type 2 external metrics are an order of magnitude larger; any Type 2 metric is considered greater than the cost of any path internal to the AS. Use of Type 2 external metrics assumes that routing between AS'es is the major cost of routing a packet, and eliminates the need for conversion of external costs to internal link state metrics.
As an example of Type 1 external metric processing, suppose that the Routers RT7 and RT5 in Figure 2 are advertising Type 1 external metrics. For each advertised external route, the total cost from Router RT6 is calculated as the sum of the external route's advertised cost and the distance from Router RT6 to the advertising router. When two routers are advertising the same external destination, RT6 picks the advertising router providing the minimum total cost. RT6 then sets the next hop to the external destination equal to the next hop that would be used when routing packets to the chosen advertising router.
In Figure 2, both Router RT5 and RT7 are advertising an external route to destination Network N12. Router RT7 is preferred since it is advertising N12 at a distance of 10 (8+2) to Router RT6, which is better than Router RT5's 14 (6+8). Table 3 shows the entries that are added to the routing table when external routes are examined:
Destination Next Hop Distance
__________________________________
N12 RT10 10
N13 RT5 14
N14 RT5 14
N15 RT10 17
Table 3: The portion of Router RT6's routing table listing external destinations.
Processing of Type 2 external metrics is simpler. The AS boundary router advertising the smallest external metric is chosen, regardless of the internal distance to the AS boundary router. Suppose in our example both Router RT5 and Router RT7 were advertising Type 2 external routes. Then all traffic destined for Network N12 would be forwarded to Router RT7, since 2 < 8. When several equal-cost Type 2 routes exist, the internal distance to the advertising routers is used to break the tie.
Both Type 1 and Type 2 external metrics can be present in the AS at the same time. In that event, Type 1 external metrics always take precedence.
This section has assumed that packets destined for external destinations are always routed through the advertising AS boundary router. This is not always desirable. For example, suppose in Figure 2 there is an additional router attached to Network N6, called Router RTX. Suppose further that RTX does not participate in OSPF
routing, but does exchange BGP information with the AS boundary router RT7. Then, Router RT7 would end up advertising OSPF external routes for all destinations that should be routed to RTX. An extra hop will sometimes be introduced if packets for these destinations need always be routed first to Router RT7 (the advertising router).
To deal with this situation, the OSPF protocol allows an AS boundary router to specify a "forwarding address" in its AS- external-LSAs. In the above example, Router RT7 would specify RTX's IP address as the "forwarding address" for all those destinations whose packets should be routed directly to RTX.
The "forwarding address" has one other application. It enables routers in the Autonomous System's interior to function as "route servers". For example, in Figure 2 the router RT6 could become a route server, gaining external routing information through a combination of static configuration and external routing protocols. RT6 would then start advertising itself as an AS boundary router, and would originate a collection of OSPF AS-external-LSAs. In each AS- external-LSA, Router RT6 would specify the correct Autonomous System exit point to use for the destination through appropriate setting of the LSA's "forwarding address" field.
The above discussion has been simplified by considering only a single route to any destination. In reality, if multiple equal-cost routes to a destination exist, they are all discovered and used. This requires no conceptual changes to the algorithm, and its discussion is postponed until we consider the tree-building process in more detail.
With equal cost multipath, a router potentially has several available next hops towards any given destination.
OSPF allows collections of contiguous networks and hosts to be grouped together. Such a group, together with the routers having interfaces to any one of the included networks, is called an area. Each area runs a separate copy of the basic link-state routing algorithm. This means that each area has its own link-state database and corresponding graph, as explained in the previous section.
The topology of an area is invisible from the outside of the area. Conversely, routers internal to a given area know nothing of the detailed topology external to the area. This isolation of knowledge enables the protocol to effect a marked reduction in routing traffic
as compared to treating the entire Autonomous System as a single link-state domain.
With the introduction of areas, it is no longer true that all routers in the AS have an identical link-state database. A router actually has a separate link-state database for each area it is connected to. (Routers connected to multiple areas are called area border routers). Two routers belonging to the same area have, for that area, identical area link-state databases.
Routing in the Autonomous System takes place on two levels, depending on whether the source and destination of a packet reside in the same area (intra-area routing is used) or different areas (inter-area routing is used). In intra-area routing, the packet is routed solely on information obtained within the area; no routing information obtained from outside the area can be used. This protects intra-area routing from the injection of bad routing information. We discuss inter-area routing in Section 3.2.
The OSPF backbone is the special OSPF Area 0 (often written as Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP addresses). The OSPF backbone always contains all area border routers. The backbone is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous. However, it need not be physically contiguous; backbone connectivity can be established/maintained through the configuration of virtual links.
Virtual links can be configured between any two backbone routers that have an interface to a common non-backbone area. Virtual links belong to the backbone. The protocol treats two routers joined by a virtual link as if they were connected by an unnumbered point-to- point backbone network. On the graph of the backbone, two such routers are joined by arcs whose costs are the intra-area distances between the two routers. The routing protocol traffic that flows along the virtual link uses intra-area routing only.
When routing a packet between two non-backbone areas the backbone is used. The path that the packet will travel can be broken up into three contiguous pieces: an intra-area path from the source to an area border router, a backbone path between the source and destination areas, and then another intra-area path to the destination. The algorithm finds the set of such paths that have the smallest cost.
Looking at this another way, inter-area routing can be pictured as forcing a star configuration on the Autonomous System, with the backbone as hub and each of the non-backbone areas as spokes.
The topology of the backbone dictates the backbone paths used between areas. The topology of the backbone can be enhanced by adding virtual links. This gives the system administrator some control over the routes taken by inter-area traffic.
The correct area border router to use as the packet exits the source area is chosen in exactly the same way routers advertising external routes are chosen. Each area border router in an area summarizes for the area its cost to all networks external to the area. After the SPF tree is calculated for the area, routes to all inter-area destinations are calculated by examining the summaries of the area border routers.
Before the introduction of areas, the only OSPF routers having a specialized function were those advertising external routing information, such as Router RT5 in Figure 2. When the AS is split into OSPF areas, the routers are further divided according to function into the following four overlapping categories:
Internal routers
A router with all directly connected networks belonging to the
same area. These routers run a single copy of the basic routing
algorithm.
Area border routers
A router that attaches to multiple areas. Area border routers run
multiple copies of the basic algorithm, one copy for each attached
area. Area border routers condense the topological information of
their attached areas for distribution to the backbone. The
backbone in turn distributes the information to the other areas.
Backbone routers
A router that has an interface to the backbone area. This
includes all routers that interface to more than one area (i.e.,
area border routers). However, backbone routers do not have to be
area border routers. Routers with all interfaces connecting to
the backbone area are supported.
AS boundary routers
A router that exchanges routing information with routers belonging
to other Autonomous Systems. Such a router advertises AS external
routing information throughout the Autonomous System. The paths
to each AS boundary router are known by every router in the AS.
This classification is completely independent of the previous
classifications: AS boundary routers may be internal or area
border routers, and may or may not participate in the backbone.
Figure 6 shows a sample area configuration. The first area consists of networks N1-N4, along with their attached routers RT1-RT4. The second area consists of networks N6-N8, along with their attached routers RT7, RT8, RT10 and RT11. The third area consists of networks N9-N11 and Host H1, along with their attached routers RT9, RT11 and RT12. The third area has been configured so that networks N9-N11 and Host H1 will all be grouped into a single route, when advertised external to the area (see Section 3.5 for more details).
In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are internal routers. Routers RT3, RT4, RT7, RT10 and RT11 are area border routers. Finally, as before, Routers RT5 and RT7 are AS boundary routers.
Figure 7 shows the resulting link-state database for the Area 1. The figure completely describes that area's intra-area routing.
. + .
. | 3+---+ . N12 N14
. N1|--|RT1|\ 1 . \ N13 /
. | +---+ \ . 8\ |8/8
. + \ ____ . \|/
. / \ 1+---+8 8+---+6
. * N3 *---|RT4|------|RT5|--------+
. \____/ +---+ +---+ |
. + / \ . |7 |
. | 3+---+ / \ . | |
. N2|--|RT2|/1 1\ . |6 |
. | +---+ +---+8 6+---+ |
. + |RT3|------|RT6| |
. +---+ +---+ |
. 2/ . Ia|7 |
. / . | |
. +---------+ . | |
.Area 1 N4 . | |
. N11 . | |
. +---------+ . | |
. | . | | N12
. |3 . Ib|5 |6 2/
. +---+ . +----+ +---+/
. |RT9| . .........|RT10|.....|RT7|---N15.
. +---+ . . +----+ +---+ 9 .
. |1 . . + /3 1\ |1 .
. _|__ . . | / \ __|_ .
. / \ 1+----+2 |/ \ / \ .
. * N9 *------|RT11|----| * N6 * .
. \____/ +----+ | \____/ .
. | . . | | .
. |1 . . + |1 .
. +--+ 10+----+ . . N8 +---+ .
. |H1|-----|RT12| . . |RT8| .
. +--+SLIP +----+ . . +---+ .
. |2 . . |4 .
. | . . | .
. +---------+ . . +--------+ .
. N10 . . N7 .
. . .Area 2 .
.Area 3 . ................................
Figure 6: A sample OSPF area configuration
It also shows the complete view of the internet for the two internal routers RT1 and RT2. It is the job of the area border routers, RT3 and RT4, to advertise into Area 1 the distances to all destinations external to the area. These are indicated in Figure 7 by the dashed stub routes. Also, RT3 and RT4 must advertise into Area 1 the location of the AS boundary routers RT5 and RT7. Finally, AS- external-LSAs from RT5 and RT7 are flooded throughout the entire AS, and in particular throughout Area 1. These LSAs are included in Area 1's database, and yield routes to Networks N12-N15.
Routers RT3 and RT4 must also summarize Area 1's topology for distribution to the backbone. Their backbone LSAs are shown in Table 4. These summaries show which networks are contained in Area 1 (i.e., Networks N1-N4), and the distance to these networks from the routers RT3 and RT4 respectively.
The link-state database for the backbone is shown in Figure 8. The set of routers pictured are the backbone routers. Router RT11 is a backbone router because it belongs to two areas. In order to make the backbone connected, a virtual link has been configured between Routers R10 and R11.
The area border routers RT3, RT4, RT7, RT10 and RT11 condense the routing information of their attached non-backbone areas for distribution via the backbone; these are the dashed stubs that appear in Figure 8. Remember that the third area has been configured to condense Networks N9-N11 and Host H1 into a single route. This yields a single dashed line for networks N9-N11 and Host H1 in Figure 8. Routers RT5 and RT7 are AS boundary routers; their externally derived information also appears on the graph in Figure 8 as stubs.
Network RT3 adv. RT4 adv.
_____________________________
N1 4 4
N2 4 4
N3 1 1
N4 2 3
Table 4: Networks advertised to the backbone
by Routers RT3 and RT4.
|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |7 |N3|
----- -------------------
RT1| | | | | | |0 |
RT2| | | | | | |0 |
RT3| | | | | | |0 |
* RT4| | | | | | |0 |
* RT5| | |14|8 | | | |
T RT7| | |20|14| | | |
O N1|3 | | | | | | |
* N2| |3 | | | | | |
* N3|1 |1 |1 |1 | | | |
N4| | |2 | | | | |
Ia,Ib| | |20|27| | | |
N6| | |16|15| | | |
N7| | |20|19| | | |
N8| | |18|18| | | |
N9-N11,H1| | |29|36| | | |
N12| | | | |8 |2 | |
N13| | | | |8 | | |
N14| | | | |8 | | |
N15| | | | | |9 | |
Figure 7: Area 1's Database.
Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X.
**FROM**
|RT|RT|RT|RT|RT|RT|RT
|3 |4 |5 |6 |7 |10|11|
------------------------
RT3| | | |6 | | | |
RT4| | |8 | | | | |
RT5| |8 | |6 |6 | | |
RT6|8 | |7 | | |5 | |
RT7| | |6 | | | | |
* RT10| | | |7 | | |2 |
* RT11| | | | | |3 | |
T N1|4 |4 | | | | | |
O N2|4 |4 | | | | | |
* N3|1 |1 | | | | | |
* N4|2 |3 | | | | | |
Ia| | | | | |5 | |
Ib| | | |7 | | | |
N6| | | | |1 |1 |3 |
N7| | | | |5 |5 |7 |
N8| | | | |4 |3 |2 |
N9-N11,H1| | | | | | |11|
N12| | |8 | |2 | | |
N13| | |8 | | | | |
N14| | |8 | | | | |
N15| | | | |9 | | |
Figure 8: The backbone's database.
Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X.
The backbone enables the exchange of summary information between area border routers. Every area border router hears the area summaries from all other area border routers. It then forms a picture of the distance to all networks outside of its area by examining the collected LSAs, and adding in the backbone distance to each advertising router.
Again using Routers RT3 and RT4 as an example, the procedure goes as follows: They first calculate the SPF tree for the backbone. This gives the distances to all other area border routers. Also noted are the distances to networks (Ia and Ib) and AS boundary routers (RT5 and RT7) that belong to the backbone. This calculation is shown in Table 5.
Next, by looking at the area summaries from these area border routers, RT3 and RT4 can determine the distance to all networks outside their area. These distances are then advertised internally to the area by RT3 and RT4. The advertisements that Router RT3 and RT4 will make into Area 1 are shown in Table 6. Note that Table 6 assumes that an area range has been configured for the backbone which groups Ia and Ib into a single LSA.
The information imported into Area 1 by Routers RT3 and RT4 enables an internal router, such as RT1, to choose an area border router intelligently. Router RT1 would use RT4 for traffic to Network N6, RT3 for traffic to Network N10, and would load share between the two for traffic to Network N8.
dist from dist from
RT3 RT4
__________________________________
to RT3 * 21
to RT4 22 *
to RT7 20 14
to RT10 15 22
to RT11 18 25
__________________________________
to Ia 20 27
to Ib 15 22
__________________________________
to RT5 14 8
to RT7 20 14
Table 5: Backbone distances calculated
by Routers RT3 and RT4.
Destination RT3 adv. RT4 adv.
_________________________________
Ia,Ib 20 27
N6 16 15
N7 20 19
N8 18 18
N9-N11,H1 29 36
_________________________________
RT5 14 8
RT7 20 14
Table 6: Destinations advertised into Area 1
by Routers RT3 and RT4.
Router RT1 can also determine in this manner the shortest path to the AS boundary routers RT5 and RT7. Then, by looking at RT5 and RT7's AS-external-LSAs, Router RT1 can decide between RT5 or RT7 when sending to a destination in another Autonomous System (one of the networks N12-N15).
Note that a failure of the line between Routers RT6 and RT10 will cause the backbone to become disconnected. Configuring a virtual link between Routers RT7 and RT10 will give the backbone more connectivity and more resistance to such failures.
OSPF attaches an IP address mask to each advertised route. The mask indicates the range of addresses being described by the particular route. For example, a summary-LSA for the destination 128.185.0.0 with a mask of 0xffff0000 actually is describing a single route to the collection of destinations 128.185.0.0 - 128.185.255.255. Similarly, host routes are always advertised with a mask of 0xffffffff, indicating the presence of only a single destination.
Including the mask with each advertised destination enables the implementation of what is commonly referred to as variable-length subnetting. This means that a single IP class A, B, or C network number can be broken up into many subnets of various sizes. For example, the network 128.185.0.0 could be broken up into 62 variable-sized subnets: 15 subnets of size 4K, 15 subnets of size 256, and 32 subnets of size 8. Table 7 shows some of the resulting network addresses together with their masks.
Network address IP address mask Subnet size
_______________________________________________
128.185.16.0 0xfffff000 4K
128.185.1.0 0xffffff00 256
128.185.0.8 0xfffffff8 8
Table 7: Some sample subnet sizes.
There are many possible ways of dividing up a class A, B, and C network into variable sized subnets. The precise procedure for doing so is beyond the scope of this specification. This specification however establishes the following guideline: When an IP packet is forwarded, it is always forwarded to the network that is the best match for the packet's destination. Here best match is synonymous with the longest or most specific match. For example, the default
route with destination of 0.0.0.0 and mask 0x00000000 is always a match for every IP destination. Yet it is always less specific than any other match. Subnet masks must be assigned so that the best match for any IP destination is unambiguous.
Attaching an address mask to each route also enables the support of IP supernetting. For example, a single physical network segment could be assigned the [address,mask] pair [192.9.4.0,0xfffffc00]. The segment would then be single IP network, containing addresses from the four consecutive class C network numbers 192.9.4.0 through 192.9.7.0. Such addressing is now becoming commonplace with the advent of CIDR (see [Ref10]).
In order to get better aggregation at area boundaries, area address ranges can be employed (see Section C.2 for more details). Each address range is defined as an [address,mask] pair. Many separate networks may then be contained in a single address range, just as a subnetted network is composed of many separate subnets. Area border routers then summarize the area contents (for distribution to the backbone) by advertising a single route for each address range. The cost of the route is the maximum cost to any of the networks falling in the specified range.
For example, an IP subnetted network might be configured as a single OSPF area. In that case, a single address range could be configured: a class A, B, or C network number along with its natural IP mask. Inside the area, any number of variable sized subnets could be defined. However, external to the area a single route for the entire subnetted network would be distributed, hiding even the fact that the network is subnetted at all. The cost of this route is the maximum of the set of costs to the component subnets.
In some Autonomous Systems, the majority of the link-state database may consist of AS-external-LSAs. An OSPF AS-external-LSA is usually flooded throughout the entire AS. However, OSPF allows certain areas to be configured as "stub areas". AS-external-LSAs are not flooded into/throughout stub areas; routing to AS external destinations in these areas is based on a (per-area) default only. This reduces the link-state database size, and therefore the memory requirements, for a stub area's internal routers.
In order to take advantage of the OSPF stub area support, default routing must be used in the stub area. This is accomplished as follows. One or more of the stub area's area border routers must advertise a default route into the stub area via summary-LSAs. These summary defaults are flooded throughout the stub area, but no
further. (For this reason these defaults pertain only to the particular stub area). These summary default routes will be used for any destination that is not explicitly reachable by an intra-area or inter-area path (i.e., AS external destinations).
An area can be configured as a stub when there is a single exit point from the area, or when the choice of exit point need not be made on a per-external-destination basis. For example, Area 3 in Figure 6 could be configured as a stub area, because all external traffic must travel though its single area border router RT11. If Area 3 were configured as a stub, Router RT11 would advertise a default route for distribution inside Area 3 (in a summary-LSA), instead of flooding the AS-external-LSAs for Networks N12-N15 into/throughout the area.
The OSPF protocol ensures that all routers belonging to an area agree on whether the area has been configured as a stub. This guarantees that no confusion will arise in the flooding of AS-external-LSAs.
There are a couple of restrictions on the use of stub areas. Virtual links cannot be configured through stub areas. In addition, AS boundary routers cannot be placed internal to stub areas.
OSPF does not actively attempt to repair area partitions. When an area becomes partitioned, each component simply becomes a separate area. The backbone then performs routing between the new areas. Some destinations reachable via intra-area routing before the partition will now require inter-area routing.
However, in order to maintain full routing after the partition, an address range must not be split across multiple components of the area partition. Also, the backbone itself must not partition. If it does, parts of the Autonomous System will become unreachable. Backbone partitions can be repaired by configuring virtual links (see Section 15).
Another way to think about area partitions is to look at the Autonomous System graph that was introduced in Section 2. Area IDs can be viewed as colors for the graph's edges.[1] Each edge of the graph connects to a network, or is itself a point-to-point network. In either case, the edge is colored with the network's Area ID.
A group of edges, all having the same color, and interconnected by vertices, represents an area. If the topology of the Autonomous System is intact, the graph will have several regions of color, each color being a distinct Area ID.
When the AS topology changes, one of the areas may become partitioned. The graph of the AS will then have multiple regions of the same color (Area ID). The routing in the Autonomous System will continue to function as long as these regions of same color are connected by the single backbone region.
A separate copy of OSPF's basic routing algorithm runs in each area. Routers having interfaces to multiple areas run multiple copies of the algorithm. A brief summary of the routing algorithm follows.
When a router starts, it first initializes the routing protocol data structures. The router then waits for indications from the lower- level protocols that its interfaces are functional.
A router then uses the OSPF's Hello Protocol to acquire neighbors. The router sends Hello packets to its neighbors, and in turn receives their Hello packets. On broadcast and point-to-point networks, the router dynamically detects its neighboring routers by sending its Hello packets to the multicast address AllSPFRouters. On non- broadcast networks, some configuration information may be necessary in order to discover neighbors. On broadcast and NBMA networks the Hello Protocol also elects a Designated router for the network.
The router will attempt to form adjacencies with some of its newly acquired neighbors. Link-state databases are synchronized between pairs of adjacent routers. On broadcast and NBMA networks, the Designated Router determines which routers should become adjacent.
Adjacencies control the distribution of routing information. Routing updates are sent and received only on adjacencies.
A router periodically advertises its state, which is also called link state. Link state is also advertised when a router's state changes. A router's adjacencies are reflected in the contents of its LSAs. This relationship between adjacencies and link state allows the protocol to detect dead routers in a timely fashion.
LSAs are flooded throughout the area. The flooding algorithm is reliable, ensuring that all routers in an area have exactly the same link-state database. This database consists of the collection of LSAs originated by each router belonging to the area. From this database each router calculates a shortest-path tree, with itself as root. This shortest-path tree in turn yields a routing table for the protocol.
The previous section described the operation of the protocol within a single area. For intra-area routing, no other routing information is pertinent. In order to be able to route to destinations outside of the area, the area border routers inject additional routing information into the area. This additional information is a distillation of the rest of the Autonomous System's topology.
This distillation is accomplished as follows: Each area border router is by definition connected to the backbone. Each area border router summarizes the topology of its attached non-backbone areas for transmission on the backbone, and hence to all other area border routers. An area border router then has complete topological information concerning the backbone, and the area summaries from each of the other area border routers. From this information, the router calculates paths to all inter-area destinations. The router then advertises these paths into its attached areas. This enables the area's internal routers to pick the best exit router when forwarding traffic inter-area destinations.
Routers that have information regarding other Autonomous Systems can flood this information throughout the AS. This external routing information is distributed verbatim to every participating router. There is one exception: external routing information is not flooded into "stub" areas (see Section 3.6).
To utilize external routing information, the path to all routers advertising external information must be known throughout the AS (excepting the stub areas). For that reason, the locations of these AS boundary routers are summarized by the (non-stub) area border routers.
The OSPF protocol runs directly over IP, using IP protocol 89. OSPF does not provide any explicit fragmentation/reassembly support. When fragmentation is necessary, IP fragmentation/reassembly is used. OSPF protocol packets have been designed so that large protocol packets can generally be split into several smaller protocol packets. This practice is recommended; IP fragmentation should be avoided whenever possible.
Routing protocol packets should always be sent with the IP TOS field set to 0. If at all possible, routing protocol packets should be given preference over regular IP data traffic, both when being sent
and received. As an aid to accomplishing this, OSPF protocol packets should have their IP precedence field set to the value Internetwork Control (see [Ref5]).
All OSPF protocol packets share a common protocol header that is described in Appendix A. The OSPF packet types are listed below in Table 8. Their formats are also described in Appendix A.
Type Packet name
Protocol function
__________________________________________________________
1 Hello Discover/maintain neighbors
2 Database Description Summarize database contents
3 Link State Request Database download
4 Link State Update Database update
5 Link State Ack Flooding acknowledgment
Table 8: OSPF packet types.
OSPF's Hello protocol uses Hello packets to discover and maintain neighbor relationships. The Database Description and Link State Request packets are used in the forming of adjacencies. OSPF's reliable update mechanism is implemented by the Link State Update and Link State Acknowledgment packets.
Each Link State Update packet carries a set of new link state advertisements (LSAs) one hop further away from their point of origination. A single Link State Update packet may contain the LSAs of several routers. Each LSA is tagged with the ID of the originating router and a checksum of its link state contents. Each LSA also has a type field; the different types of OSPF LSAs are listed below in Table 9.
OSPF routing packets (with the exception of Hellos) are sent only over adjacencies. This means that all OSPF protocol packets travel a single IP hop, except those that are sent over virtual adjacencies. The IP source address of an OSPF protocol packet is one end of a router adjacency, and the IP destination address is either the other end of the adjacency or an IP multicast address.
LS LSA LSA description
type name
________________________________________________________
1 Router-LSAs Originated by all routers.
This LSA describes
the collected states of the
router's interfaces to an
area. Flooded throughout a
single area only.
________________________________________________________
2 Network-LSAs Originated for broadcast
and NBMA networks by
the Designated Router. This
LSA contains the
list of routers connected
to the network. Flooded
throughout a single area only.
________________________________________________________
3,4 Summary-LSAs Originated by area border
routers, and flooded through-
out the LSA's associated
area. Each summary-LSA
describes a route to a
destination outside the area,
yet still inside the AS
(i.e., an inter-area route).
Type 3 summary-LSAs describe
routes to networks. Type 4
summary-LSAs describe
routes to AS boundary routers.
________________________________________________________
5 AS-external-LSAs Originated by AS boundary
routers, and flooded through-
out the AS. Each
AS-external-LSA describes
a route to a destination in
another Autonomous System.
Default routes for the AS can
also be described by
AS-external-LSAs.
Table 9: OSPF link state advertisements (LSAs).
An implementation of OSPF requires the following pieces of system support:
Timers
Two different kind of timers are required. The first kind, called
"single shot timers", fire once and cause a protocol event to be
processed. The second kind, called "interval timers", fire at
continuous intervals. These are used for the sending of packets
at regular intervals. A good example of this is the regular
broadcast of Hello packets. The granularity of both kinds of
timers is one second.
Interval timers should be implemented to avoid drift. In some
router implementations, packet processing can affect timer
execution. When multiple routers are attached to a single
network, all doing broadcasts, this can lead to the
synchronization of routing packets (which should be avoided). If
timers cannot be implemented to avoid drift, small random amounts
should be added to/subtracted from the interval timer at each
firing.
IP multicast
Certain OSPF packets take the form of IP multicast datagrams.
Support for receiving and sending IP multicast datagrams, along
with the appropriate lower-level protocol support, is required.
The IP multicast datagrams used by OSPF never travel more than one
hop. For this reason, the ability to forward IP multicast
datagrams is not required. For information on IP multicast, see
[Ref7].
Variable-length subnet support
The router's IP protocol support must include the ability to
divide a single IP class A, B, or C network number into many
subnets of various sizes. This is commonly called variable-length
subnetting; see Section 3.5 for details.
IP supernetting support
The router's IP protocol support must include the ability to
aggregate contiguous collections of IP class A, B, and C networks
into larger quantities called supernets. Supernetting has been
proposed as one way to improve the scaling of IP routing in the
worldwide Internet. For more information on IP supernetting, see
[Ref10].
Lower-level protocol support
The lower level protocols referred to here are the network access
protocols, such as the Ethernet data link layer. Indications must
be passed from these protocols to OSPF as the network interface
goes up and down. For example, on an ethernet it would be
valuable to know when the ethernet transceiver cable becomes
unplugged.
Non-broadcast lower-level protocol support
On non-broadcast networks, the OSPF Hello Protocol can be aided by
providing an indication when an attempt is made to send a packet
to a dead or non-existent router. For example, on an X.25 PDN a
dead neighboring router may be indicated by the reception of a
X.25 clear with an appropriate cause and diagnostic, and this
information would be passed to OSPF.
List manipulation primitives
Much of the OSPF functionality is described in terms of its
operation on lists of LSAs. For example, the collection of LSAs
that will be retransmitted to an adjacent router until
acknowledged are described as a list. Any particular LSA may be
on many such lists. An OSPF implementation needs to be able to
manipulate these lists, adding and deleting constituent LSAs as
necessary.
Tasking support
Certain procedures described in this specification invoke other
procedures. At times, these other procedures should be executed
in-line, that is, before the current procedure is finished. This
is indicated in the text by instructions to execute a procedure.
At other times, the other procedures are to be executed only when
the current procedure has finished. This is indicated by
instructions to schedule a task.
The OSPF protocol defines several optional capabilities. A router indicates the optional capabilities that it supports in its OSPF Hello packets, Database Description packets and in its LSAs. This enables routers supporting a mix of optional capabilities to coexist in a single Autonomous System.
Some capabilities must be supported by all routers attached to a specific area. In this case, a router will not accept a neighbor's Hello Packet unless there is a match in reported capabilities (i.e., a capability mismatch prevents a neighbor relationship from forming). An example of this is the ExternalRoutingCapability (see below).
Other capabilities can be negotiated during the Database Exchange process. This is accomplished by specifying the optional capabilities in Database Description packets. A capability mismatch with a neighbor in this case will result in only a subset of the link state database being exchanged between the two neighbors.
The routing table build process can also be affected by the presence/absence of optional capabilities. For example, since the optional capabilities are reported in LSAs, routers incapable of certain functions can be avoided when building the shortest path tree.
The OSPF optional capabilities defined in this memo are listed below. See Section A.2 for more information.
ExternalRoutingCapability
Entire OSPF areas can be configured as "stubs" (see Section 3.6).
AS-external-LSAs will not be flooded into stub areas. This
capability is represented by the E-bit in the OSPF Options field
(see Section A.2). In order to ensure consistent configuration of
stub areas, all routers interfacing to such an area must have the
E-bit clear in their Hello packets (see Sections 9.5 and 10.5).
The OSPF protocol is described herein in terms of its operation on various protocol data structures. The following list comprises the top-level OSPF data structures. Any initialization that needs to be done is noted. OSPF areas, interfaces and neighbors also have associated data structures that are described later in this specification.
Router ID
A 32-bit number that uniquely identifies this router in the AS.
One possible implementation strategy would be to use the smallest
IP interface address belonging to the router. If a router's OSPF
Router ID is changed, the router's OSPF software should be
restarted before the new Router ID takes effect. In this case the
router should flush its self-originated LSAs from the routing
domain (see Section 14.1) before restarting, or they will persist
for up to MaxAge minutes.
Area structures
Each one of the areas to which the router is connected has its own
data structure. This data structure describes the working of the
basic OSPF algorithm. Remember that each area runs a separate
copy of the basic OSPF algorithm.
Backbone (area) structure
The OSPF backbone area is responsible for the dissemination of
inter-area routing information.
Virtual links configured
The virtual links configured with this router as one endpoint. In
order to have configured virtual links, the router itself must be
an area border router. Virtual links are identified by the Router
ID of the other endpoint -- which is another area border router.
These two endpoint routers must be attached to a common area,
called the virtual link's Transit area. Virtual links are part of
the backbone, and behave as if they were unnumbered point-to-point
networks between the two routers. A virtual link uses the intra-
area routing of its Transit area to forward packets. Virtual
links are brought up and down through the building of the
shortest-path trees for the Transit area.
List of external routes
These are routes to destinations external to the Autonomous
System, that have been gained either through direct experience
with another routing protocol (such as BGP), or through
configuration information, or through a combination of the two
(e.g., dynamic external information to be advertised by OSPF with
configured metric). Any router having these external routes is
called an AS boundary router. These routes are advertised by the
router into the OSPF routing domain via AS-external-LSAs.
List of AS-external-LSAs
Part of the link-state database. These have originated from the
AS boundary routers. They comprise routes to destinations
external to the Autonomous System. Note that, if the router is
itself an AS boundary router, some of these AS-external-LSAs have
been self-originated.
The routing table
Derived from the link-state database. Each entry in the routing
table is indexed by a destination, and contains the destination's
cost and a set of paths to use in forwarding packets to the
destination. A path is described by its type and next hop. For
more information, see Section 11.
Figure 9 shows the collection of data structures present in a typical router. The router pictured is RT10, from the map in Figure 6. Note that Router RT10 has a virtual link configured to Router RT11, with Area 2 as the link's Transit area. This is indicated by the dashed line in Figure 9. When the virtual link becomes active, through the building of the shortest path tree for Area 2, it becomes an interface to the backbone (see the two backbone interfaces depicted in Figure 9).
+----+
|RT10|------+
+----+ \+-------------+
/ \ |Routing Table|
/ \ +-------------+
/ \
+------+ / \ +--------+
|Area 2|---+ +---|Backbone|
+------+***********+ +--------+
/ \ * / \
/ \ * / \
+---------+ +---------+ +------------+ +------------+
|Interface| |Interface| |Virtual Link| |Interface Ib|
| to N6 | | to N8 | | to RT11 | +------------+
+---------+ +---------+ +------------+ |
/ \ | | |
/ \ | | |
+--------+ +--------+ | +-------------+ +------------+
|Neighbor| |Neighbor| | |Neighbor RT11| |Neighbor RT6|
| RT8 | | RT7 | | +-------------+ +------------+
+--------+ +--------+ |
|
+-------------+
|Neighbor RT11|
+-------------+
Figure 9: Router RT10's Data structures
The area data structure contains all the information used to run the basic OSPF routing algorithm. Each area maintains its own link-state database. A network belongs to a single area, and a router interface connects to a single area. Each router adjacency also belongs to a single area.
The OSPF backbone is the special OSPF area responsible for disseminating inter-area routing information.
The area link-state database consists of the collection of router- LSAs, network-LSAs and summary-LSAs that have originated from the area's routers. This information is flooded throughout a single area only. The list of AS-external-LSAs (see Section 5) is also considered to be part of each area's link-state database.
Area ID
A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
reserved for the backbone.
List of area address ranges
In order to aggregate routing information at area boundaries, area
address ranges can be employed. Each address range is specified by
an [address,mask] pair and a status indication of either Advertise
or DoNotAdvertise (see Section 12.4.3).
Associated router interfaces
This router's interfaces connecting to the area. A router
interface belongs to one and only one area (or the backbone). For
the backbone area this list includes all the virtual links. A
virtual link is identified by the Router ID of its other endpoint;
its cost is the cost of the shortest intra-area path through the
Transit area that exists between the two routers.
List of router-LSAs
A router-LSA is generated by each router in the area. It
describes the state of the router's interfaces to the area.
List of network-LSAs
One network-LSA is generated for each transit broadcast and NBMA
network in the area. A network-LSA describes the set of routers
currently connected to the network.
List of summary-LSAs
Summary-LSAs originate from the area's area border routers. They
describe routes to destinations internal to the Autonomous System,
yet external to the area (i.e., inter-area destinations).
Shortest-path tree
The shortest-path tree for the area, with this router itself as
root. Derived from the collected router-LSAs and network-LSAs by
the Dijkstra algorithm (see Section 16.1).
TransitCapability
This parameter indicates whether the area can carry data traffic
that neither originates nor terminates in the area itself. This
parameter is calculated when the area's shortest-path tree is
built (see Section 16.1, where TransitCapability is set to TRUE if
and only if there are one or more fully adjacent virtual links
using the area as Transit area), and is used as an input to a
subsequent step of the routing table build process (see Section
16.3). When an area's TransitCapability is set to TRUE, the area
is said to be a "transit area".
ExternalRoutingCapability
Whether AS-external-LSAs will be flooded into/throughout the area.
This is a configurable parameter. If AS-external-LSAs are
excluded from the area, the area is called a "stub". Within stub
areas, routing to AS external destinations will be based solely on
a default summary route. The backbone cannot be configured as a
stub area. Also, virtual links cannot be configured through stub
areas. For more information, see Section 3.6.
StubDefaultCost
If the area has been configured as a stub area, and the router
itself is an area border router, then the StubDefaultCost
indicates the cost of the default summary-LSA that the router
should advertise into the area. See Section 12.4.3 for more
information.
Unless otherwise specified, the remaining sections of this document refer to the operation of the OSPF protocol within a single area.
OSPF creates adjacencies between neighboring routers for the purpose of exchanging routing information. Not every two neighboring routers will become adjacent. This section covers the generalities involved in creating adjacencies. For further details consult Section 10.
The Hello Protocol is responsible for establishing and maintaining neighbor relationships. It also ensures that communication between neighbors is bidirectional. Hello packets are sent periodically out all router interfaces. Bidirectional communication is indicated when the router sees itself listed in the neighbor's Hello Packet. On broadcast and NBMA networks, the Hello Protocol elects a Designated Router for the network.
The Hello Protocol works differently on broadcast networks, NBMA networks and Point-to-MultiPoint networks. On broadcast networks, each router advertises itself by periodically multicasting Hello Packets. This allows neighbors to be discovered dynamically. These Hello Packets contain the router's view of the Designated Router's identity, and the list of routers whose Hello Packets have been seen recently.
On NBMA networks some configuration information may be necessary for the operation of the Hello Protocol. Each router that may potentially become Designated Router has a list of all other routers attached to the network. A router, having Designated Router potential, sends Hello Packets to all other potential Designated Routers when its interface to the NBMA network first becomes operational. This is an attempt to find the Designated Router for the network. If the router itself is elected Designated Router, it begins sending Hello Packets to all other routers attached to the network.
On Point-to-MultiPoint networks, a router sends Hello Packets to all neighbors with which it can communicate directly. These neighbors may be discovered dynamically through a protocol such as Inverse ARP (see [Ref14]), or they may be configured.
After a neighbor has been discovered, bidirectional communication ensured, and (if on a broadcast or NBMA network) a Designated Router elected, a decision is made regarding whether or not an adjacency should be formed with the neighbor (see Section 10.4). If an adjacency is to be formed, the first step is to synchronize the neighbors' link-state databases. This is covered in the next section.
In a link-state routing algorithm, it is very important for all routers' link-state databases to stay synchronized. OSPF simplifies this by requiring only adjacent routers to remain synchronized. The synchronization process begins as soon as the routers attempt to bring up the adjacency. Each router describes its database by sending a sequence of Database Description packets to its neighbor. Each Database Description Packet describes a set of LSAs belonging to the router's database. When the neighbor sees an LSA that is more recent than its own database copy, it makes a note that this newer LSA should be requested.
This sending and receiving of Database Description packets is called the "Database Exchange Process". During this process, the two routers form a master/slave relationship. Each Database Description
Packet has a sequence number. Database Description Packets sent by the master (polls) are acknowledged by the slave through echoing of the sequence number. Both polls and their responses contain summaries of link state data. The master is the only one allowed to retransmit Database Description Packets. It does so only at fixed intervals, the length of which is the configured per-interface constant RxmtInterval.
Each Database Description contains an indication that there are more packets to follow --- the M-bit. The Database Exchange Process is over when a router has received and sent Database Description Packets with the M-bit off.
During and after the Database Exchange Process, each router has a list of those LSAs for which the neighbor has more up-to-date instances. These LSAs are requested in Link State Request Packets. Link State Request packets that are not satisfied are retransmitted at fixed intervals of time RxmtInterval. When the Database Description Process has completed and all Link State Requests have been satisfied, the databases are deemed synchronized and the routers are marked fully adjacent. At this time the adjacency is fully functional and is advertised in the two routers' router-LSAs.
The adjacency is used by the flooding procedure as soon as the Database Exchange Process begins. This simplifies database synchronization, and guarantees that it finishes in a predictable period of time.
Every broadcast and NBMA network has a Designated Router. The Designated Router performs two main functions for the routing protocol:
The Designated Router is elected by the Hello Protocol. A router's Hello Packet contains its Router Priority, which is configurable on a per-interface basis. In general, when a router's interface to a network first becomes functional, it checks to see whether there is currently a Designated Router for the network. If there is, it accepts that Designated Router, regardless of its Router Priority. (This makes it harder to predict the identity of the Designated Router, but ensures that the Designated Router changes less often. See below.) Otherwise, the router itself becomes Designated Router if it has the highest Router Priority on the network. A more detailed (and more accurate) description of Designated Router election is presented in Section 9.4.
The Designated Router is the endpoint of many adjacencies. In order to optimize the flooding procedure on broadcast networks, the Designated Router multicasts its Link State Update Packets to the address AllSPFRouters, rather than sending separate packets over each adjacency.
Section 2 of this document discusses the directed graph
representation of an area. Router nodes are labelled with their
Router ID. Transit network nodes are actually labelled with the IP
address of their Designated Router. It follows that when the
Designated Router changes, it appears as if the network node on the
graph is replaced by an entirely new node. This will cause the
network and all its attached routers to originate new LSAs. Until
the link-state databases again converge, some temporary loss of
connectivity may result. This may result in ICMP unreachable
messages being sent in response to data traffic. For that reason,
the Designated Router should change only infrequently. Router
Priorities should be configured so that the most dependable router on
a network eventually becomes Designated Router.
In order to make the transition to a new Designated Router smoother, there is a Backup Designated Router for each broadcast and NBMA network. The Backup Designated Router is also adjacent to all routers on the network, and becomes Designated Router when the previous Designated Router fails. If there were no Backup Designated Router, when a new Designated Router became necessary, new adjacencies would have to be formed between the new Designated Router and all other routers attached to the network. Part of the adjacency forming process is the synchronizing of link-state databases, which can potentially take quite a long time. During this time, the network would not be available for transit data traffic. The Backup Designated obviates the need to form these adjacencies, since they already exist. This means the period of disruption in transit
traffic lasts only as long as it takes to flood the new LSAs (which announce the new Designated Router).
The Backup Designated Router does not generate a network-LSA for the network. (If it did, the transition to a new Designated Router would be even faster. However, this is a tradeoff between database size and speed of convergence when the Designated Router disappears.)
The Backup Designated Router is also elected by the Hello Protocol. Each Hello Packet has a field that specifies the Backup Designated Router for the network.
In some steps of the flooding procedure, the Backup Designated Router plays a passive role, letting the Designated Router do more of the work. This cuts down on the amount of local routing traffic. See Section 13.3 for more information.
An adjacency is bound to the network that the two routers have in common. If two routers have multiple networks in common, they may have multiple adjacencies between them.
One can picture the collection of adjacencies on a network as forming an undirected graph. The vertices consist of routers, with an edge joining two routers if they are adjacent. The graph of adjacencies describes the flow of routing protocol packets, and in particular Link State Update Packets, through the Autonomous System.
Two graphs are possible, depending on whether a Designated Router is elected for the network. On physical point-to-point networks, Point-to-MultiPoint networks and virtual links, neighboring routers become adjacent whenever they can communicate directly. In contrast, on broadcast and NBMA networks only the Designated Router and the Backup Designated Router become adjacent to all other routers attached to the network.
These graphs are shown in Figure 10. It is assumed that Router RT7 has become the Designated Router, and Router RT3 the Backup Designated Router, for the Network N2. The Backup Designated Router performs a lesser function during the flooding procedure than the Designated Router (see Section 13.3). This is the reason for the dashed lines connecting the Backup Designated Router RT3.
+---+ +---+
|RT1|------------|RT2| o---------------o
+---+ N1 +---+ RT1 RT2
RT7
o---------+
+---+ +---+ +---+ /|\ |
|RT7| |RT3| |RT4| / | \ |
+---+ +---+ +---+ / | \ |
| | | / | \ |
+-----------------------+ RT5o RT6o oRT4 |
| | N2 * * * |
+---+ +---+ * * * |
|RT5| |RT6| * * * |
+---+ +---+ *** |
o---------+
RT3
Figure 10: The graph of adjacencies
This section discusses the general processing of OSPF routing protocol packets. It is very important that the router link-state databases remain synchronized. For this reason, routing protocol packets should get preferential treatment over ordinary data packets, both in sending and receiving.
Routing protocol packets are sent along adjacencies only (with the exception of Hello packets, which are used to discover the adjacencies). This means that all routing protocol packets travel a single IP hop, except those sent over virtual links.
All routing protocol packets begin with a standard header. The sections below provide details on how to fill in and verify this standard header. Then, for each packet type, the section giving more details on that particular packet type's processing is listed.
When a router sends a routing protocol packet, it fills in the fields of the standard OSPF packet header as follows. For more details on the header format consult Section A.3.1:
Version #
Set to 2, the version number of the protocol as documented in this
specification.
Packet type
The type of OSPF packet, such as Link state Update or Hello
Packet.
Packet length
The length of the entire OSPF packet in bytes, including the
standard OSPF packet header.
Router ID
The identity of the router itself (who is originating the packet).
Area ID
The OSPF area that the packet is being sent into.
Checksum
The standard IP 16-bit one's complement checksum of the entire
OSPF packet, excluding the 64-bit authentication field. This
checksum is calculated as part of the appropriate authentication
procedure; for some OSPF authentication types, the checksum
calculation is omitted. See Section D.4 for details.
AuType and Authentication
Each OSPF packet exchange is authenticated. Authentication types
are assigned by the protocol and are documented in Appendix D. A
different authentication procedure can be used for each IP
network/subnet. Autype indicates the type of authentication
procedure in use. The 64-bit authentication field is then for use
by the chosen authentication procedure. This procedure should be
the last called when forming the packet to be sent. See Section
D.4 for details.
The IP destination address for the packet is selected as follows. On physical point-to-point networks, the IP destination is always set to the address AllSPFRouters. On all other network types (including virtual links), the majority of OSPF packets are sent as unicasts, i.e., sent directly to the other end of the adjacency. In this case, the IP destination is just the Neighbor IP address associated with the other end of the adjacency (see Section 10). The only packets not sent as unicasts are on broadcast networks; on these networks Hello packets are sent to the multicast destination AllSPFRouters, the Designated Router and its Backup send both Link State Update Packets and Link State Acknowledgment Packets to the multicast address AllSPFRouters, while all other routers send both their Link State Update and Link State Acknowledgment Packets to the multicast address AllDRouters.
Retransmissions of Link State Update packets are ALWAYS sent as unicasts.
The IP source address should be set to the IP address of the sending interface. Interfaces to unnumbered point-to-point networks have no associated IP address. On these interfaces, the IP source should be set to any of the other IP addresses belonging to the router. For this reason, there must be at least one IP address assigned to the router.[2] Note that, for most purposes, virtual links act precisely the same as unnumbered point-to-point networks. However, each virtual link does have an IP interface address (discovered during the routing table build process) which is used as the IP source when sending packets over the virtual link.
For more information on the format of specific OSPF packet types, consult the sections listed in Table 10.
Type Packet name detailed section (transmit)
_________________________________________________________
1 Hello Section 9.5
2 Database description Section 10.8
3 Link state request Section 10.9
4 Link state update Section 13.3
5 Link state ack Section 13.5
Table 10: Sections describing OSPF protocol packet transmission.
Whenever a protocol packet is received by the router it is marked with the interface it was received on. For routers that have virtual links configured, it may not be immediately obvious which interface
to associate the packet with. For example, consider the Router RT11 depicted in Figure 6. If RT11 receives an OSPF protocol packet on its interface to Network N8, it may want to associate the packet with the interface to Area 2, or with the virtual link to Router RT10 (which is part of the backbone). In the following, we assume that the packet is initially associated with the non-virtual link.[3]
In order for the packet to be accepted at the IP level, it must pass a number of tests, even before the packet is passed to OSPF for processing:
Next, the OSPF packet header is verified. The fields specified in the header must match those configured for the receiving interface. If they do not, the packet should be discarded:
(1) Match the Area ID of the receiving interface. In this case, the packet has been sent over a single hop. Therefore, the packet's IP source address is required to be on the same network as the receiving interface. This can be verified by comparing the packet's IP source address to the interface's IP address, after masking both addresses with the interface mask. This comparison should not be performed on point-to-point networks. On point-to-point networks, the interface addresses of each end of the link are assigned independently, if they are assigned at all.
(2) Indicate the backbone. In this case, the packet has
been sent over a virtual link. The receiving router
must be an area border router, and the Router ID
specified in the packet (the source router) must be the
other end of a configured virtual link. The receiving
interface must also attach to the virtual link's
configured Transit area. If all of these checks
succeed, the packet is accepted and is from now on
associated with the virtual link (and the backbone
area).
If the packet type is Hello, it should then be further processed by the Hello Protocol (see Section 10.5). All other packet types are sent/received only on adjacencies. This means that the packet must have been sent by one of the router's active neighb