OSPF Areas

In a very large AS with a large number of networks, each OSPF router must keep the LSA of every other router in its LSDB. Each router in a large OSPF AS has a large LSDB. The SPF calculation of a large LSDB can require a substantial amount of processing. Also, the resulting routing table can be very large, containing a route to each network in the AS.

In an effort to reduce the size of the LSDB and the processing overhead for the SPF tree and routing table calculation, OSPF allows the AS to be divided up into contiguous groups of networks called areas. Areas are identified through a 32-bit Area ID expressed in dotted decimal notation.

An Area ID is an administrative identifier and has no relation to an IP address or IP network ID. Area IDs are not used to reflect routing data. However, if all of the networks within an area correspond to a single subnetted network ID, the area ID can be set to reflect the network ID for administrative convenience. For example, if an area contains all of the subnets of the IP network 10.1.0.0, the area ID can be set to 10.1.0.0.

Reducing the Size of the LSDB

To keep the size of the LSDB for each router to a minimum, LSAs for an area's networks and routers are flooded within the area but not to routers outside the area. Each area becomes its own link state domain with its own LSDB.

If a router is connected to multiple areas, it has multiple LSDBs and SPF Trees. The routing table is a combination of the routing table entries of all the SPF Trees for the router as well as static routes, SNMP configured routes, and routes learned from other routing protocols.

Reducing the Size of the Routing Table

To reduce the number of entries in the routing table of OSPF routers, the networks inside of the area can be advertised outside the area using summary route advertisements. In Figure 3.18, the router on the border of Area 0.0.0.1, known as an area border router (ABR), advertises a summary of all of the networks inside Area 0.0.0.1 in the form of [Destination, Network Mask] pairs to the ABRs of Area 0.0.0.2 and Area 0.0.0.3. Through route summarization, the topology (the networks and their path costs) of an area is hidden from the rest of the AS.

Figure 3.18 OSPF AS and Areas

When the topology of an area is hidden, the rest of the AS is protected from route flapping, events that cause networks to come up or go down. If a network comes up, the event is propagated as a Link State Update and flooded through adjacencies to routers within the area. However, because all the networks within the area are advertised outside the area using summary routes, the Link State Update is not flooded outside the area.

You can view the current OSPF areas by right-clicking the OSPF routing protocol and clicking Show Areas in the Routing and Remote Access snap-in.

Backbone Area

An OSPF internetwork, whether or not it is subdivided into areas, always has at least one area called the backbone. The backbone has the reserved area ID of 0.0.0.0. The OSPF backbone area is also known as area 0.

The backbone acts as a hub for inter-area transit traffic and the distribution of routing information between areas. Inter-area traffic is routed to the backbone, then routed to the destination area, and finally routed to the destination host within the destination area (for more information, see "Inter-Area Routing" later in this chapter). Routers on the backbone also advertise the summarized routes within their areas to the other routers on the backbone. These summary advertisements are flooded into area routers. Therefore, each router in an area has a routing table that reflects the routes available within its area and the routes corresponding to the summary advertisements of the ABRs of the other areas in the AS.

For example, in Figure 3.18, router R1 advertises all of the routes (the list of address ranges) in Area 0.0.0.1 to all backbone routers (routers R2 and R3) using a summary advertisement. R1 receives summary advertisements from R2 and R3. R1 is configured with summary advertisement information for Area 0.0.0.0. Through flooding, R1 propagates that summary routing information to all of the routers within Area 0.0.0.1. For each router within Area 0.0.0.1, the summary routing information from Areas 0.0.0.0, 0.0.0.2, and 0.0.0.3 is incorporated into the calculation of the routing table.

OSPF Router Types

When an OSPF AS is subdivided into areas, the routers are classified by one or more of the categories defined in Table 3.4.

Table 3.4 OSPF Router Types

Inter-Area Routing

Routing within an area is performed by OSPF routers using the least cost path to the destination network. Because routes within an area are not summarized, each router has a route to each network within its area or areas.

Routing between areas takes the following course:

1. Routers within the source area forward the packet along the least cost path to the nearest ABR.

2. Backbone routers forward the packet along the least cost path to the nearest ABR connected to the area containing the IP address of the destination host.

3. Routers within the area containing the IP address of the destination host forward the packet along the least cost path to the destination host.

In Figure 3.19, the packet is forwarded across the routers of Area 0.0.0.1 to router R1, a backbone area router. Then, the packet is forwarded across the routers of the backbone area (Area 0.0.0.0) to Router R2. And finally, the packet is forwarded across the routers of Area 0.0.0.2 to the destination host.

Figure 3.19 Inter-Area Routing in OSPF

Note

OSPF routers do not make routing decisions based on area IDs. All routing decisions are based on the entries in the IP routing table. For example, in the inter-area routing shown in Figure 3.19, the backbone routers are not explicitly forwarding the packet to Area 0.0.0.2. They are forwarding it along the least cost path to the route that is the best match for the destination IP address in the packet.