Route Aggregation

Route aggregation allows you to aggregate one or more specific routes into a single route. Aggregate routes are only created if a more specific route of the aggregate route exists in the BGP routing table. Aggregate route configuration options provide for:

• Creating and advertising the aggregate route while at the same time suppressing the advertisement of all the more specific routes for this aggregate through route summarization.
• Retaining the advertisement of the AS-Path information for the specific routes of the aggregate. The default behavior for an aggregate route is to suppress the AS-Path information for the specific routes of the aggregate. It may be desirable to retain AS-Path information for routes in the aggregate that belong to an AS outside of the AS in which the aggregate is created.
• Enabling both route summarization and advertisement of the AS-Path information for the specific routes of the aggregate.
• Creating and advertising an aggregate while at the same time suppressing only those specific routes that match clauses in the applied route-map. Prefixes contained in the aggregate route that are not specifically matched in the route-map are not suppressed. You can not use this option in conjunction with route summarization.
• Creating and advertising an aggregate, while at the same time allow specifying in a route-map which AS path information is retained in the aggregate. This option is used in conjunction with retaining the advertisement of the AS-Path information for specific routes of the aggregate. You can not use this option in conjunction with route summarization.
• Creating and advertising an aggregate, while at the same time allowing for the modifying of aggregate route attributes specified in the route-map.

Some routes that are members of an aggregation may be rejected to avoid potential loops. Use the no-reject option to no longer reject any routes in the aggregation.

The following example creates and advertises aggregate route 200.51.0.0/22 and suppresses the advertisement of all the more specific routes for this aggregate:

System(su-config)->router bgp 65151
System(su-config-bgp)->aggregate-address 200.51.0.0/22 summary
System(su-config-bgp)->

The following example sets the MED attribute to 50 for routes in aggregate route 200.51.0.0/22 using route-map attrmap1:

System(su-config)->route-map bgp attrmap1 permit 10
System(su-config-route-map-bgp)->set med 50
System(su-config-route-map-bgp)->exit
System(su-config)->show route-map attrmap1
 route-map bgp attrmap1 permit 10
  set med 50
System(su-config)->router bgp 65151
System(su-config-bgp)->aggregate-address 200.51.0.0/22 attribute-map attrmap1
System(su-config-bgp)->

The following example retains AS-path information for routes 200.51.1.0/24 using route-map advmap1 in aggregate route 200.51.0.0/22:

System(su-config)->ip prefix-list advlist1 permit seq 1 200.51.1.0/24
System(su-config)->route-map bgp advmap1
System(su-config-route-map-bgp)->match prefix-list advlist1
System(su-config-route-map-bgp)->exit
System(su-config)->router bgp 65151
System(su-config-bgp)->aggregate-address 200.51.0.0/22 advertise-map advmap1
System(su-config-bgp)->