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Aggregating Routes

Aggregation applies only to routes that are present in the BGP routing table. BGP advertises an aggregate route only if the routing table contains at least one prefix that is more specific than the aggregate. You aggregate IPv4 routes by specifying the aggregate IP address, and IPv6 routes by specifying the aggregate IPv6 prefix.

Figure 17 illustrates an IPv4 network structure where you might use aggregation. The following commands configure router LA and router Snakes so that router Snakes advertises an IPv4 aggregate route, 172.24.0.0/16, for the more specific prefixes 172.24.1.0/24, 172.24.2.0/24, and 172.24.24.0/21.

Figure 17: Configuring Aggregate Addresses

Image g013175.gif

To configure router LA:



host1(config)#router bgp 873 

host1(config-router)#neighbor 10.2.2.4 remote-as
873 

host1(config-router)#network 172.24.1.0 mask
255.255.255.0 

host1(config-router)#network 172.24.2.0 mask
255.255.255.0

To configure router SanJose:



host2(config)#router bgp 873 

host2(config-router)#neighbor 10.2.2.3 remote-as
873 

host2(config-router)#neighbor 10.5.5.1 remote-as
17 

host2(config-router)#network 172.24.24.0 mask
255.255.248.0 

host2(config-router)#aggregate-address 172.24.0.0
255.255.224.0

As configured above, router SanJose advertises the more specific routes as well as the aggregate route to router Boston. Alternatively, you can use the summary-only option to configure router SanJose to suppress the more specific routes and advertise only the aggregate route:



host2(config)#router bgp 873 

host2(config-router)#neighbor 10.2.2.3 remote-as
873 

host2(config-router)#neighbor 10.5.5.1 remote-as
17 

host2(config-router)#network 172.24.24.0 mask
255.255.248.0 

host2(config-router)#aggregate-address 172.24.0.0
255.255.224.0  summary-only

Each of these configurations sets the atomic-aggregate attribute in the aggregate route. This attribute informs recipients that the route is an aggregate and must not be deaggregated into more specific routes.

Aggregate routes discard the path information carried in the original routes. To preserve the paths, you must use the as-set option. This option creates an AS-Set that consists of all the AS numbers traversed by the summarized paths. The AS-Set is enclosed within curly brackets; for example, {3, 2}. Each AS number appears only once, even if it appears in more than one of the original paths. If you use the as-set option, the atomic-aggregate attribute is not set for the aggregated route. The following commands configure router SanJose to aggregate the routes while preserving the path information:



host2(config)#router bgp 873 

host2(config-router)#neighbor 10.2.2.3 remote-as
873 

host2(config-router)#neighbor 10.5.5.1 remote-as
17 

host2(config-router)#network 172.24.24.0 mask
255.255.248.0 

host2(config-router)#aggregate-address 172.24.0.0
255.255.224.0  summary-only as-set

If you do not want to aggregate all more specific routes, you can use a route map to limit aggregation. Consider Figure 17 again. Suppose you do not want router SanJose to aggregate prefix 172.24.48.0/20. The following commands show how you can configure a route map on router SanJose to match this prefix, and how to invoke the route map with the advertise-map option:



host2(config)#router bgp 873 

host2(config-router)#neighbor 10.2.2.3 remote-as
873 

host2(config-router)#neighbor 10.5.5.1 remote-as
17 

host2(config-router)#neighbor 10.2.2.3 route-map
lmt_agg in 

host2(config-router)#network 172.24.24.0 mask
255.255.248.0 

host2(config-router)#aggregate-address 172.24.0.0
255.255.224.0  advertise-map lmt_agg 

host2(config-router)#exit 

host2(config)#route-map lmt_agg permit 10  

host2(config-route-map)#match ip address 1 

host2(config-route-map)#exit 

host2(config)#access-list 1 permit 172.24.48.0
0.240.255.255

You can use the attribute-map option to configure attributes for the aggregated route. In Figure 17, suppose that router LA has been configured to set the community attribute for route 172.24.160.0/19 to no-export. This attribute is passed along to router SanJose and preserved when the aggregate route is created. As a result, the aggregate route is not advertised outside the AS. The following commands demonstrate how to configure router SanJose to prevent the aggregate from not being advertised:



host2(config)#router bgp 873 

host2(config-router)#neighbor 10.2.2.3 remote-as
873 

host2(config-router)#neighbor 10.5.5.1 remote-as
17 

host2(config-router)#network 172.24.24.0 mask
255.255.248.0 

host2(config-router)#aggregate-address 172.24.0.0
255.255.224.0  attribute-map conf_agg_att 

host2(config-router)#exit 

host2(config)#route-map conf_agg_att permit
10  

host2(config-route-map)#set community no-export

aggregate-address

Use to create an aggregate entry in a BGP routing table that summarizes more specific routes.
For IPv4 routes, you must specify an aggregate IP address (address) and aggregate IP mask (mask). For IPv6 routes, you must specify an aggregate IPv6 prefix (ipv6Prefix).

The optional as-set keyword preserves path information by creating an AS-Set that contains all the AS numbers traversed by the aggregated routes.

Note: Do not use the as-set keyword when you have many paths to aggregate. If you do, the aggregated route is continually withdrawn and reupdated as AS-path reachability information changes for the summarized routes.

The summary-only keyword advertises only the aggregate route; it suppresses the advertisement of all more specific routes. Contrast with the suppress-map keyword.

The suppress-map keyword enables you to specify a route map to filter particular routes covered by the aggregate that will be suppressed. Contrast with the summary-only keyword.

Note: If you want to suppress advertisements only to certain neighbors, you can—with caution—use the neighbor distribute-list command. If a more specific route leaks out, all BGP speakers will prefer that route over the less specific aggregate you are generating (using longest-match routing).

The advertise-map keyword enables you to specify the advertise-map-tag, a string of up to 32 characters that identifies the route map that sets the routes to create AS-Set origin communities.
The attribute-map keyword enables you to specify the attribute-map-tag, a string of up to 32 characters that identifies the route map that sets the attributes of the aggregate route.
This command takes effect immediately.
Use the no version to remove the aggregate route entry from the routing table.
See aggregate-address.

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