BGP Selective Aggregation : suppress-map

Overview

Selective aggregation means that when routes are aggregated in BGP, some pre-aggregation routes are selected and advertised together with the aggregated routes, rather than all or nothing, where all pre-aggregation routes are sent or none at all.

This section describes the suppress-map option of the aggregate-address command for selective aggregation. suppress-map option allows you to determine the pre-aggregation routes that are not advertised.

aggregate-address command : suppress-map option

Selective aggregation with the suppress-map option is considered on the basis that all default pre-aggregation routes are advertised. The suppress-map option allows you to stop some of the pre-aggregation routes from being advertised along with the aggregation routes.

The syntax of the support-map command is as follows.

Note that the route map is specified after suppress-map, but it is easy to get confused with the route map configuration suppress-map is an option to “stop” the pre-aggregation routes. Therefore, the pre-aggregation routes that are permit in the route-map after suppress-map are no longer advertised. Conversely, pre-aggregation routes that you want to advertise along with the aggregation route are configured to be denied in the route-map.

Selective aggregation with suppress-map configuration example

Consider the specific configuration in “Figure: Selective Aggregation with suppress-map”. Generate an aggregate route that aggregates the following four BGP routes.

• 192.168.0.0/24
• 192.168.1.0/24
• 192.168.2.0/24
• 192.168.3.0/24

Aggregating these four routes with the longest subnet mask is “192.168.0.0/22”. Along with this aggregate route, “192.168.1.0/24” and “192.168.3.0/24” are also advertised. In other words, stop advertising 192.168.0.0/24 and 192.168.2.0/24 of the pre-aggregation route. The configuration of the suppress-map is as follows.

access-list 1 permit 192.168.0.0
access-list 1 permit 192.168.2.0
!
route-map SUPP-MAP permit 10
 match ip address 1
!
router bgp 
 aggregate-address 192.168.0.0 255.255.252.0 suppress-map SUPP-MAP

The routes matching the sequence 10 match condition in the route-map “SUPP-MAP” are 192.168.0.0/24 and 192.168.2.0/24. Since the route map SUPP-MAP is specified in suppress-map, 192.168.0.0/24 and 192.168.2.0/24 advertisements will be suppressed.

The route map “SUPP-MAP” still has an implicit deny any at the end.192.168.1.0/24と192.168.3.0/24. Therefore, the advertisement of these two pre-aggregation routes will not be suppressed and will be advertised to the neighbor along with the aggregate route.

set action is not required for route-map specified the suppress-map.

And note the BGP table when suppress-map is configured. If you suppress the advertisement of a particular pre-aggregation route with suppress-map, the BGP table will be a mixture of “*(valid)” and “s(suppressed)”.

R1#sh ip bgp 
BGP table version is 8, local router ID is 100.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.0.0/22   0.0.0.0                            32768 i
s> 192.168.0.0/24   0.0.0.0                  0         32768 i
*> 192.168.1.0/24   0.0.0.0                  0         32768 i
s> 192.168.2.0/24   0.0.0.0                  0         32768 i
*> 192.168.3.0/24   0.0.0.0                  0         32768 i

Summary