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Advertising a Route Only When Another Route is Absent

You can use the non-exist-map keyword with the neighbor advertise-map command to advertise a route only when the BGP routing table does not contain some other particular route.

In the network shown in Figure 19, router R2 has established BGP sessions with both router R1 and router R3. The plan is for router R2 to send peergroup1 an advertisement for the route to prefix 10.10.30.0/24 only if the route to prefix 172.24.20.0/24 is not present in the BGP routing table. Alternatively, if router R2 has not received a route to prefix 172.21.30.0 from router R3, then router R2 advertises to peergroup1 the route to prefix 10.10.20.0. In this sample network, router R3 advertises neither of the routes to router R2. Consequently, router R2 advertises both 10.10.20.0/24 and 10.10.30.0/24 to peergroup1.

Figure 19: Advertising a Route When Another Route is Absent

Image g016471.gif

The following commands configure router R2:



host1(config)#router bgp 200 

host1(config-router)#neighbor peergroup1 peer-group 

host1(config-router)#neighbor peergroup1 remote-as
100 

host1(config-router)#neighbor 10.6.6.2 peer-group
peergroup1 

host1(config-router)#neighbor 10.7.3.2 peer-group
peergroup1 

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.

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