Figure 20: Advertising A Default Route When Another Route Is Present - Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual

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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 69, 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 The default route is withdrawn if this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

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
Chapter 1: Configuring BGP Routing
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