According to RFC 2328 – OSPF Version 2, an area border router (ABR) is a router attached to multiple areas, and has the main function to provide its attached areas with Type-3 and Type-4 LSAs that describe routes and ASBRs in other areas, as well as perform actual inter-area routing.
Based on RFC 2328, RT3 will identify itself as an ABR by setting the B bit in its Router-LSA, and generates Type-3 Summary-LSAs for the destinations in its directly attached areas into Areas 1 and 2 – originates Summary-LSAs for Area 1 to Area 2; and vice versa.
The Cisco’s implementation of ABRs is based on RFC 3509 – Alternative Implementations of OSPF Area Border Routers, which redefines an ABR as a router that has more than one area actively attached and one of them is the backbone area! RT3 is no longer considered as an ABR; this is proven when the output of the show ip ospf database router self-originate EXEC command on RT3 does not have the “Area Border Router” message as in RT1 and RT4. Cisco using this approach to prevent loops due to the origination of Summary-LSAs in situations where a router considers itself as an ABR but doesn’t have an active backbone connection. Deployment of the Cisco’s implementation of ABRs based on RFC 3509 is fully compatible and can coexist with standard OSPF domains without problems.
Deployment of either OSPF-standard or RFC3509-based ABRs can lead to unexpected routing asymmetry as discussed in this section.
RT2 will have the inter-area route to 192.168.2.0/24 via RT1 (ABR); while RT5 will have the inter-area route to 192.168.1.0/24 via RT4 (ABR). Since RT4 is only reachable via RT3, the traffic initiated from 192.168.2.0/24 and destined to 192.168.1.0/24 will pass through RT3. Due to RT3 has an intra-area route to 192.168.1.0/24 via RT2, RT3 will route the traffic directly to RT2 instead of RT4, as intra-area routes are always preferred over inter-area routes.
Note: The OSPF cost from RT3 to 192.168.1.0/24 via RT2 is 9 while the OSPF cost via RT4 is 7.
The return traffic from 192.168.1.0/24 back to 192.168.2.0/24 will pass through RT2 and routed to RT1 instead of RT3, as RT2 do not have any inter-area routes via RT3. As a result, routing asymmetry occurs and the more optimal path through the backbone area is not being used as the traffic from 192.168.1.0/24 to 192.168.2.0/24 will go through the backbone area while traffic from 192.168.2.0/24 to 192.168.1.0/24 will cross the areas directly through RT3. Below shows the output on RT3 which proves that asymmetric routing has occurred:
RT3#sh int | in is up|rate FastEthernet0/0 is up, line protocol is up 30 second input rate 18000 bits/sec, 20 packets/sec 30 second output rate 0 bits/sec, 0 packets/sec Ethernet1/0 is up, line protocol is up 30 second input rate 0 bits/sec, 0 packets/sec 30 second output rate 18000 bits/sec, 20 packets/sec Ethernet1/1 is up, line protocol is up 30 second input rate 18000 bits/sec, 20 packets/sec 30 second output rate 18000 bits/sec, 20 packets/sec RT3#
The RFC3509-based-ABR RT3 is not really functioning as an ABR as it does not originate Summary-LSAs into its attached areas, but inter-area traffic can still pass through it.