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.