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 online.

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

    In either case above, the routing switch with the higher router ID floods the AS-external-LSAs and the other routing-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.