Advertising A Default Route Only When Another Route Is Present - Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual

host1(config)#route-map alternatetoPG1 permit 10
host1(config-route-map)#match ip address allow
host1(config-route-map)#exit
!
!Configure route map to match an alternate trigger route
!
host1(config)#access-list test permit 172.21.30.0 0.0.0.255
host1(config)#route-map condition2 permit 10
host1(config-route-map)#match ip address test
host1(config-route-map)#match as-path 1
host1(config-route-map)#exit
In this configuration, the condition1 route map has a sequence number of five, placing
it high in the list of all configured advertise route maps for this peer group within the
same address family. The condition2 route map has no sequence number configured,
thus placing it at the bottom of the route map list.
In this configuration, the condition1 route map has a sequence number of ten. Route
maps configured for this peer group within the same address family and a lower
sequence number are processed before the condition1 route map. The condition2
route map has no sequence number configured, thus giving the route map the
sequence number of ten and ensuring that condition2 is processed after the condition1
route map.

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 on page 70, 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.
Chapter 1: Configuring BGP Routing
Advertising Routes
69