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

JunosE 11.2.x BGP and MPLS Configuration Guide
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host1(config-router)#neighbor 10.4.4.2 peer-group peergroup1
host1(config-router)#neighbor peergroup1 advertise-map advertisetoPG1 non-exist-map
condition1 sequence 5
host1(config-router)#neighbor peer-group1 advertise-map alternatetoPG1 non-exist-map
condition2
host1(config-router)#exit
host1(config)#ip as-path access-list 1 permit ^300
!
!Configure route map to send one route to peergroup1
!
host1(config)#access-list 77 permit 10.10.30.0 0.0.0.255
host1(config)#route-map advertisetoPG1 permit 10
host1(config-route-map)#match ip address 77
host1(config-route-map)#exit
!
!Configure route map to match one trigger route
!
host1(config)#access-list 70 permit 172.24.20.0 0.0.0.255
host1(config)#route-map condition1 permit 10
host1(config-route-map)#match ip address 70
host1(config-route-map)#exit
!
!Configure route map to send alternate route to peergroup1
!
host1(config)#access-list allow permit 10.10.20.0 0.0.0.255
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
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