BGP aggregate-address

Background Information

1. Sometimes we want to reduce the size of the BGP table.
2. BGP gives us a way to advertise an aggregate-address.

Summary

1. aggregate-address to define an aggregate address.  If at least one component prefix exists in the BGP table, the aggregate-address will be advertised.  By default, the component prefixes are not suppressed and will be advertised as they normally do.  This is different from the way IGPs summarize.
2. The as-set attribute advertises the component prefix AS as a set.  When a router sees itself in the as-set, it will drop the update.  We sometimes need this to prevent routing information loops.  Keep in mind a component route flap will cause updates.
3. The summary-only attribute suppresses all of the component prefixes.
4. The suppress-map suppresses individual component prefixes.  You can then use unsuppress-map on the neighbor to conditionally unsuppress prefixes that have been suppressed.
5. Use a route-map or an attribute-map to set the attributes of the aggregate.
6. Use an advertise-map to specify which component prefix attributes to use for the aggregate.
7. An inject-map can be used to advertise a more specific route if a more general route exists.

router bgp 100

 address-family ipv4

   aggregate-address IP MASK

     route-map

     summary-only

     as-set

     attribute-map

     advertise-map

     suppress-map

 bgp inject-map exist-map ! Inject a more specific route if aggregate exists

 neighbor X unsuppress-map ROUTE-MAP