host1(config)#access-list 77 permit 10.10.20.0 0.0.0.255
host1(config)#route-map advertisetoR1 permit 10
host1(config-route-map)#match ip address 77
host1(config-route-map)#exit
!
!Configure route map to match one trigger route from R3
!
host1(config)#ip as-path access-list 1 permit ^300
host1(config)#access-list 70 permit 172.24.19.0 0.0.0.255
host1(config)#route-map trigger1 permit 10
host1(config-route-map)#match ip address 70
host1(config-route-map)#match as-path 1
host1(config-route-map)#exit
!
!Configure route map to send alternate route to R1
!
host1(config)#access-list test permit 10.10.30.0 0.0.0.255
host1(config)#route-map alternatetoR1 permit 10
host1(config-route-map)#match ip address test
host1(config-route-map)#exit
!
!Configure route map to match alternate route from R3
!
host1(config)#access-list check permit 172.24.20.0 0.0.0.255
host1(config)#route-map trigger2 permit 10
host1(config-route-map)#match ip address check
host1(config-route-map)#exit
The match as-path clause in the route map referenced by the exist-map keyword
ensures that router 2 sends router 1 the route to prefix 10.10.20.0 only if a route to
172.24.19.0/24 with an AS path of 300 is present in the BGP routing table. Similarly,
you can impose additional restraints by including any other match clause that is
supported by an existing outbound policy.
In this configuration, the condition1 route map has a sequence number of ten.
Advertise route maps configured for this peer 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 a sequence number of 15 and ensuring that condition2 is processed after the
condition1 route map.
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 on page 68, 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
Advertising Routes â– 67
Chapter 1: Configuring BGP Routing
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