3. Splitting the AS into Areas 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 topological 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 topological database. A router actually has a separate topological 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 topological 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. 3.1. The backbone of the Autonomous System The backbone consists of those networks not contained in any area, their attached routers, and those routers that belong to multiple areas. The backbone must be contiguous. It is possible to define areas in such a way that the backbone is no longer contiguous. In this case the system administrator must restore backbone connectivity by configuring 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 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. The backbone is responsible for distributing routing information between areas. The backbone itself has all of the properties of an area. The topology of the backbone is invisible to each of the areas, while the backbone itself knows nothing of the topology of the areas. 3.2. Inter-area routing When routing a packet between two 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 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 other networks are calculated by examining the summaries of the area border routers. 3.3. Classification of 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. Routers with only backbone interfaces also belong to this category. 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 and an additional copy for the backbone. 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. 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 connected to the backbone are considered to be internal routers.
AS boundary routers A router that exchanges routing information with routers belonging to other Autonomous Systems. Such a router has AS external routes that are advertised throughout the Autonomous System. The path to each AS boundary router is 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. 3.4. A sample area configuration 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 topological database for the Area 1. The figure completely describes that area's intra-area routing. 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, external advertisements from RT5 and RT7 are flooded throughout the entire AS, and in particular throughout Area 1. These advertisements 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 advertisements 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.
........................... . + . . | 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
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. The topological 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. Again, Routers RT3, RT4, RT7, RT10 and RT11 are area border routers. As Routers RT3 and RT4 did above, they have condensed the routing information of their attached 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. 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 advertisements, 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.
**FROM** |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| | |15|22| | | | N6| | |16|15| | | | N7| | |20|19| | | | N8| | |18|18| | | | N9-N11,H1| | |19|16| | | | 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| | | | | | |1 | 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.
Area border dist from dist from router 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. Note that Table 6 assumes that an area range has been configured for the backbone which groups Ia and Ib into a single advertisement. 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. Destination RT3 adv. RT4 adv. _________________________________ Ia,Ib 15 22 N6 16 15 N7 20 19 N8 18 18 N9-N11,H1 19 26 _________________________________ 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 external advertisements, 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. Also, a virtual link between RT7 and RT10 would allow a much shorter path between the third area (containing N9) and the router RT7, which is advertising a good route to external network N12. 3.5. IP subnetting support 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 advertisement for the destination 18.104.22.168 with a mask of 0xffff0000 actually is describing a single route to the collection of destinations 22.214.171.124 - 126.96.36.199. 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 188.8.131.52 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 _______________________________________________ 184.108.40.206 0xfffff000 4K 220.127.116.11 0xffffff00 256 18.104.22.168 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. The OSPF area concept is modelled after an IP subnetted network. OSPF areas have been loosely defined to be a collection of networks. In actuality, an OSPF area is specified to be a list of address ranges (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 minimum cost to any of the networks falling in the specified range. For example, an IP subnetted network can be configured as a single OSPF area. In that case, the area would be defined as a single address range: 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. 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 minimum of the set of costs to the component subnets. 3.6. Supporting stub areas In some Autonomous Systems, the majority of the topological database may consist of AS external advertisements. An OSPF AS external advertisement is usually flooded throughout the entire AS. However, OSPF allows certain areas to be configured as "stub areas". AS external advertisements 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 topological 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 link advertisements. 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 match 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 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 link advertisement), instead of flooding the AS external advertisements 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 advertisements. 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. 3.7. Partitions of 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. In the previous section, an area was described as a list of address ranges. Any particular address range must still be completely contained in a single component of the area partition. This has to do with the way the area contents are summarized to the backbone. 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. 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.
4. Functional Summary 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 is necessary in order to discover neighbors. On all multi-access networks (broadcast or non-broadcast), 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. Topological databases are synchronized between pairs of adjacent routers. On multi-access networks, the Designated Router determines which routers should become adjacent. Adjacencies control the distribution of routing protocol packets. Routing protocol packets are sent and received only on adjacencies. In particular, distribution of topological database updates proceeds along 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 link state advertisements. This relationship between adjacencies and link state allows the protocol to detect dead routers in a timely fashion. Link state advertisements are flooded throughout the area. The flooding algorithm is reliable, ensuring that all routers in an area have exactly the same topological database. This database consists of the collection of link state advertisements received from 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.
4.1. Inter-area routing 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 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 destinations not contained in its attached areas. 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 to destinations in other areas. 4.2. AS external routes 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. 4.3. Routing protocol packets 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 [RFC 791]). 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 one hop further away from their point of origination. A single Link State Update packet may contain the link state advertisements of several routers. Each advertisement is tagged with the ID of the originating router and a checksum of its link state contents. The five different types of OSPF link state advertisements are listed below in Table 9. As mentioned above, OSPF routing packets (with the exception of Hellos) are sent only over adjacencies. Note that 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
LS Advertisement Advertisement description type name _________________________________________________________ 1 Router links Originated by all routers. advertisements This advertisement describes the collected states of the router's interfaces to an area. Flooded throughout a single area only. _________________________________________________________ 2 Network links Originated for multi-access advertisements networks by the Designated Router. This advertisement contains the list of routers connected to the network. Flooded throughout a single area only. _________________________________________________________ 3,4 Summary link Originated by area border advertisements routers, and flooded through- out the advertisement's associated area. Each summary link advertisement describes a route to a destination out- side the area, yet still inside the AS (i.e., an inter-area route). Type 3 advertisements describe routes to networks. Type 4 advertisements describe routes to AS boundary routers. _________________________________________________________ 5 AS external link Originated by AS boundary advertisements routers, and flooded through- out the AS. Each AS external link advertisement describes a route to a destination in another Autonomous System. Default routes for the AS can also be described by AS external link advertisements. Table 9: OSPF link state advertisements.
end of the adjacency or an IP multicast address. 4.4. Basic implementation requirements 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 (on broadcast networks). 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 timer interval 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 [RFC 1112]. 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 [RFC 1519]. 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 Remember that non-broadcast networks are multi-access networks such as a X.25 PDN. On these networks, the Hello Protocol can be aided by providing an indication to OSPF 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 link state advertisements. For example, the collection of advertisements that will be retransmitted to an adjacent router until acknowledged are described as a list. Any particular advertisement may be on many such lists. An OSPF implementation needs to be able to manipulate these lists, adding and deleting constituent advertisements 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. 4.5. Optional OSPF capabilities 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 link state advertisements. 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 link state advertisements 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 link state advertisements, routers incapable of certain functions can be avoided when building the shortest path tree. An example of this is the TOS routing capability (see below). The current OSPF optional capabilities 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 advertisements 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). TOS capability All OSPF implementations must be able to calculate separate routes based on IP Type of Service. However, to save routing table space and processing resources, an OSPF router can be configured to ignore TOS when forwarding packets. In this case, the router calculates routes for TOS 0 only. This capability is represented by the T-bit in the OSPF options field (see Section A.2). TOS-capable routers will attempt to avoid non-TOS-capable routers when calculating non-zero TOS paths.
5. Protocol Data Structures The OSPF protocol is described in this specification 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. Before restarting in order to change its Router ID, the router should flush its self-originated link state advertisements from the routing domain (see Section 14.1), 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 algorithm. Remember that each area runs a separate copy of the basic algorithm. Backbone (area) structure The basic algorithm operates on the backbone as if it were an area. For this reason the backbone is represented as an area structure. 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 EGP), 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 link advertisements. List of AS external link advertisements Part of the topological 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 link advertisements have been self-originated. The routing table Derived from the topological database. Each destination that the router can forward to is represented by a cost and a set of paths. A path is described by its type and next hop. For more information, see Section 11. TOS capability This item indicates whether the router will calculate separate routes based on TOS. This is a configurable parameter. For more information, see Sections 4.5 and 16.9. 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). 6. The Area Data Structure The area data structure contains all the information used to run the basic routing algorithm. Each area maintains its own topological 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 has all the properties of an area. For that reason it is also represented by an area data structure. Note that some items in the structure apply differently to the backbone than to non-backbone areas.
+----+ |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 topological (or link state) database consists of the collection of router links, network links and summary link advertisements that have originated from the area's routers. This information is flooded throughout a single area only. The list of AS external link advertisements (see Section 5) is also considered to be part of each area's topological database. Area ID A 32-bit number identifying the area. 0.0.0.0 is reserved for the Area ID of the backbone. If assigning subnetted networks as separate areas, the IP network number could be used as the Area ID. List of component address ranges The address ranges that define the area. Each address range is
specified by an [address,mask] pair and a status indication of either Advertise or DoNotAdvertise (see Section 12.4.3). Each network is then assigned to an area depending on the address range that it falls into (specified address ranges are not allowed to overlap). As an example, if an IP subnetted network is to be its own separate OSPF area, the area is defined to consist of a single address range - an IP network number with its natural (class A, B or C) mask. 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 structure 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 links advertisements A router links advertisement is generated by each router in the area. It describes the state of the router's interfaces to the area. List of network links advertisements One network links advertisement is generated for each transit multi-access network in the area. A network links advertisement describes the set of routers currently connected to the network. List of summary link advertisements Summary link advertisements originate from the area's area border routers. They describe routes to destinations internal to the Autonomous System, yet external to the area. Shortest-path tree The shortest-path tree for the area, with this router itself as root. Derived from the collected router links and network links advertisements by the Dijkstra algorithm (see Section 16.1). AuType The type of authentication used for this area. Authentication types are defined in Appendix D. All OSPF packet exchanges are authenticated. Different authentication schemes may be used in different areas. TransitCapability Set to TRUE if and only if there are one or more active virtual links using the area as a Transit area. Equivalently, 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, and is used as an input to a subsequent step of the routing table build process (see Section 16.3). ExternalRoutingCapability Whether AS external advertisements will be flooded into/throughout the area. This is a configurable parameter. If AS external advertisements are excluded from the area, the area is called a "stub". Internal to 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 link that the router should advertise into the area. There can be a separate cost configured for each IP TOS. See Section 12.4.3 for more information. Unless otherwise specified, the remaining sections of this document refer to the operation of the protocol in a single area. 7. Bringing Up Adjacencies 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. 7.1. The Hello Protocol 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 multi-access networks, the Hello Protocol elects a Designated Router for the network. Among other things, the Designated
Router controls what adjacencies will be formed over the network (see below). The Hello Protocol works differently on broadcast networks, as compared to non-broadcast 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 non-broadcast networks some configuration information is 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 non- broadcast 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. After a neighbor has been discovered, bidirectional communication ensured, and (if on a multi-access 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). An attempt is always made to establish adjacencies over point-to-point networks and virtual links. The first step in bringing up an adjacency is to synchronize the neighbors' topological databases. This is covered in the next section. 7.2. The Synchronization of Databases In a link-state routing algorithm, it is very important for all routers' topological 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 link state advertisements belonging to the router's database. When the neighbor sees a link state advertisement that is more recent than its own database copy, it makes a note that this newer advertisement 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 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 link state advertisements for which the neighbor has more up-to-date instances. These advertisements 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' link state advertisements. 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. 7.3. The Designated Router Every multi-access network has a Designated Router. The Designated Router performs two main functions for the routing protocol: o The Designated Router originates a network links advertisement on behalf of the network. This advertisement lists the set of routers (including the Designated Router itself) currently attached to the network. The Link State ID for this advertisement (see Section 12.1.4) is the IP interface address of the Designated Router. The IP network number can then be obtained by using the subnet/network mask.
o The Designated Router becomes adjacent to all other routers on the network. Since the link state databases are synchronized across adjacencies (through adjacency bring-up and then the flooding procedure), the Designated Router plays a central part in the synchronization process. 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. Multi-access 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 link state advertisements. Until the topological 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. 7.4. The Backup Designated Router In order to make the transition to a new Designated Router smoother, there is a Backup Designated Router for each multi- access 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 topological 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 link state advertisements (which announce the new Designated Router). The Backup Designated Router does not generate a network links advertisement 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. 7.5. The graph of adjacencies 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 the common network is multi-access. On physical point-to-point networks (and virtual links), the two routers joined by the network will be adjacent after their databases have been synchronized. On multi-access networks, both the Designated Router and the Backup
Designated Router are adjacent to all other routers attached to the network, and these account for all adjacencies. 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.