Juniper JUNOSE, Configuration Manual

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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 11.0.x BGP and MPLS Configuration Guide