Reducing AS external LSAs and Type-3 summary LSAs

An OSPF ASBR uses AS external LSAs to originate advertisements of a route to another routing domain, such as an RIP domain. These advertisements are
  • Flooded in the area in which the ASBR operates

  • Injected into the backbone area and then propagated to any other OSPF areas (except stub areas) within the local OSPF AS. If the AS includes an NSSA, there are two additional options:

    • If the NSSA includes an ASBR, you can suppress advertising some or all of its summarized external routes into the backbone area.

    • Replace all type-3 summary LSAs and the default external route from the backbone area with the default summary route (

In some cases, multiple ASBRs in an AS can originate equivalent external LSAs. The LSAs are equivalent when they have the same cost, the same next hop, and the same destination. In such cases, the switch optimizes OSPF by eliminating duplicate AS external LSAs. That is, the ASBR with the highest router ID floods the AS external LSAs for the external domain into the OSPF AS, while the other ASBRs flush the equivalent AS external LSAs from their databases. As a result, the overall volume of route advertisement traffic within the AS is reduced and the switches that flush the duplicate AS external LSAs have more memory for other OSPF data.

This enhancement implements the portion of RFC 2328 that describes AS external LSA reduction. This enhancement is enabled by default, requires no configuration, and cannot be disabled.