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when that route is not in the routing table. BGP conditional advertisement enables

you to control route advertisement without having to rely on only the best routes.

For example, in a multi-homed network, you might want to advertise certain prefixes

to one of the providers when a failure occurs in the peering session with a different

provider, or when there is only partial reachability to that peer.

In other cases, the advertisement to a peer of certain routes might be useful only in

the event that some other routes are present in the BGP routing table.

You can use the neighbor advertise-map command with route maps to configure

conditional advertisement of BGP routes to a peer or peer group within an address

family. BGP conditional advertisement does not create routes. The routes specified

by the route map in the neighbor advertise-map command must already be present

in the BGP routing table.

BGP conditional advertisement is supported in only the following address families:

■ Unicast IPv4

■ Unicast IPv6

■ Multicast IPv4

■ Multicast IPv6

■ VPNv4 unicast

■ VPNv6 unicast

NOTE: For VPNv4 unicast and VPNv6 unicast address families, we recommend that

you include a match extcommunity clause to match a route with a specific route

target. However, conditional advertisement in these address families can sometimes

result in unintended behaviors: advertisement of or based on an incorrect VPN route

or a non-VPN route.

BGP conditional advertisement is not supported in the following address families:

■ L2VPN

■ Route-target

■ VPLS

■ VPWS

Use the exist-map keyword when you want a route advertised only when another

route is present. The determining route must match the specified route map. If the

route map you specify with the exist-map keyword references multiple routes, only

one of those routes needs to be in the routing table to trigger the conditional

advertisement.

Use the non-exist-map keyword when you want a route advertised only when another

route is absent. The determining route must match the specified route map. If the

64 ■ Advertising Routes

JUNOSe 11.1.x BGP and MPLS Configuration Guide

Juniper BGP - CONFIGURATION GUIDE V 11.1.X User Manual

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