[Contents] [Prev] [Next] [Index] [Report an Error]

Advertising a Default Route Only When Another Route Is Present

In some circumstances, you might want to control the advertisement of a default route based on the reachability of an IGP prefix. Because conditional advertisement tracks the BGP routing table rather than the IP routing table, the prefixes that govern the advertisement (the conditional prefixes) must be present in the BGP routing table. In order to use the IGP prefix as a condition, you must import the IGP prefixes into the BGP routing table. You must also configure the origination of the default route.

In the network shown in Figure 20, router R2 has an EBGP session with router R3 and has an IGP session with router R1. Suppose you want to advertise the default route to router R3 based on the reachability of an IGP prefix, 172.55.55.0/24, on router R2.

On router R2, configure a conditional advertisement entry for the neighbor R3. The advertise map must match the default route and the route map referenced by the exist-map keyword must match the imported IGP prefix.

In case router R3 must not learn about the IGP prefix 172.55.55.0/24, you must configure an additional outbound route map to deny this prefix so that it is not advertised to router R3.

With this configuration, the default route is advertised to router R3 only when the IGP prefix 172.55.55.0/24 is reachable on router R2. The default route is withdrawn if this prefix becomes unreachable.

Figure 20: Advertising a Default Route When Another Route is Present

Image g016472.gif

The following commands configure router R2:



host1(config)#ip prefix-list default permit
0.0.0.0/0 

host1(config)#route-map default permit 10 

host1(config-route-map)#match ip address prefix-list
default 

host1(config-route-map)#exit 

host1(config)#ip prefix-list test-default
permit 172.55.0.0/16 

host1(config)#route-map test permit 10 

host1(config-route-map)#match ip address prefix-list
test-default 

host1(config-route-map)#exit 

host1(config)#route-map outbound deny 10 

host1(config-route-map)#match ip address prefix-list
test-default 

host1(config-route-map)#exit 

host1(config)#route-map outbound permit 20 

host1(config-route-map)#exit 

host1(config)#router bgp 200 

host1(config-router)#neighbor 10.12.12.2 remote-as
300 

host1(config-router)#network 172.55.55.0/24 

host1(config-router)#aggregate-address 172.55.0.0/16
summary-only 

host1(config-router)#neighbor 10.12.12.2 advertise-map
default exist-map test 

host1(config-router)#neighbor 10.12.12.2 default-originate 

host1(config-router)#neighbor 10.12.12.2 route-map
outbound out 

host1(config-router)#exit

[Contents] [Prev] [Next] [Index] [Report an Error]