Algorithm for AS external LSA reduction

The AS external LSA reduction feature behavior changes under the following conditions:
  • There is one ASBR advertising (originating) a route to the external destination, but one of the following happens:

    • A second ASBR comes on-line.

    • A second ASBR that is already on-line begins advertising an equivalent route to the same destination.

      In either of these cases, the switch with the higher router ID floods the AS external LSAs and the other switch flushes its equivalent AS external LSAs.

  • One of the ASBRs starts advertising a route that is no longer equivalent to the route the other ASBR is advertising. In this case, the ASBRs each flood AS external LSAs. Since the LSAs either no longer have the same cost or no longer have the same next-hop router, the LSAs are no longer equivalent, and the LSA reduction feature no longer applies.

  • The ASBR with the higher router ID becomes unavailable or is reconfigured so that it is no longer an ASBR. In this case, the other ASBR floods the AS external LSAs.