OSPF extent of reconvergence process
If a topology change occurs within an OSPF routing domain, OSPF must reconverge. For example, if an interface whose IP network is being advertised via OSPF goes down, the following must take place:
- the downed interface's network must be removed from the local routing table and OSPF database
- advertising updates must be sent to all neighboring routers stating that this network is no longer reachable via the local router
- any networks that were reachable via that particular interface are also removed, and update requests are sent to other neighboring OSPF routers to find alternative paths to those destinations
This reconvergence process will extend throughout the OSPF area and will reach the ABRs of the area.
Now, by default route summarization between OSPF areas on ABRs is not employed. If that is the case then each prefix within an OSPF area will have an LSA reach the ABR and be advertised into the neighboring area. This means that any changes to the OSPF area that take place will also update routing information in neighboring areas. If route summarization is employed at the ABR, then such updates into other areas can be avoided, thus reducing OSPF convergence time.