Conditional Advertisement

The conditional advertisement feature allows a service provider to advertise certain routes to a preferred subnet under normal operational conditions, while maintaining the ability to move its traffic to an alternative subnet should its preferred routes fail. The conditional advertisement feature uses two route-maps to achieve this capability:

• The non-exist-map route-map which contains match prefix-list clauses for the preferred route(s) used under normal operational conditions
• The advertise-map route-map which contains match prefix-list clauses for the alternative route(s) used only if a preferred route is not available

Should any route specified in the non-exist route-map fail (no longer exist), BGP advertises all the routes specified in the advertise route-map, otherwise routes in the advertise route-map are not advertised to the peer. The conditional advertisement feature can be used to reduce traffic within an AS.

To configure the conditional advertisement feature:

• Create two prefix lists: one used to match prefixes for the advertise route-map; a second to match prefixes for the non-exist route-map
• Create a BGP route-map with a match clause for the advertise prefix-list
• Create a BGP route-map with a match clause for the non-exist prefix-list
• Apply the two route-maps to the neighbor advertise-map command

The following example:

• Configures an advertise map prefix-list named adv-list1 and assigns it to BGP route-map adv-map1, specifying prefix 1.0.0.0/8 as the advertised prefix
• Configures a non-exist map prefix-list named non-exist-list1 and assigns it to BGP route-map non-exist-map1, specifying prefix 2.0.0.0/8 as the non-exist map prefix
• Configures a BGP advertise map for neighbor 192.168.12.112 that assigns adv-map1 as the advertise map route-map and non-exist-map1 as the non-exist map route-map

  System(su-config)->ip prefix-list adv-list1 permit seq 1 1.0.0.0/8
  System(su-config)->route-map bgp adv-map1
  System(su-config-route-map-bgp)->match adv-list1 adv-map1
  System(su-config-route-map-bgp)->exit
  System(su-config)->ip prefix-list non-exist-list1 permit seq 1 2.0.0.0/8
  System(su-config)->route-map bgp non-exist-map1
  System(su-config-route-map-bgp)->match non-exist-list1 non-exist-map1
  System(su-config-route-map-bgp)->exit
  System(su-config)->router bgp 1
  System(su-config-bgp)->neighbor 192.168.12.112 advertise-map adv-map1 non-exist-map non-exist-map1
  System(su-config-bgp)->

See Configuring Conditional Advertisement for a BGP conditional advertisement example.