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 67, 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
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
67Copyright © 2010, Juniper Networks, Inc.
Chapter 1: Configuring BGP Routing
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