Advertising A Route Only When Another Route Is Present - Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual

neighbor advertise-map
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commands supported by existing outbound policies are supported. The additional clauses
are useful when you want to match only on a specific route with a specific set of attributes.
Only the permit keyword is acted on in a match clause. The deny keyword is ignored.
Only exact matching of a prefix referenced by exist maps or non-exist maps is supported.
Consequently a range specified by the ge or le keyword in the prefix list referenced by
these route maps is ignored.
Clauses in a route map that include set commands or the match-set summary prefix-tree
command are ignored. To change the attributes of conditionally advertised routes, you
must use outbound routing policy.
If the contents of a referenced route map are changed, the new route map takes effect
automatically.
Use to specify a peer or peer group within the current address family to which routes
specified by a route map are advertised conditionally, depending on whether a second
route map is matched by some other routes in the BGP routing table.
If you specify a BGP peer group by using the peerGroupName argument, all the members
of the peer group inherit the characteristic configured with this command. This
characteristic cannot be overridden for individual members of the peer group.
This command takes effect immediately.
Example
host1(config-router-af)#neighbor 192.168.2.2 advertise-map advertiseroutes exist-map
matchroute sequence 10
Use the no version to remove the conditions set for advertising to the peer or peer group
the routes specified by the route map. Use the default version to remove the explicit
configuration from the peer or peer group and reestablish inheritance of the feature
configuration.
See neighbor advertise-map.

Advertising a Route Only When Another Route is Present

You can use the exist-map keyword with the neighbor advertise-map command to
advertise a route only when the routing table contains some other particular route.
In the network shown in Figure 18 on page 66, router 2 (R2) has established BGP sessions
with both router 1 (R1) and router 3 (R3). The plan is for router 2 to send router 1 an
advertisement for the route to prefix 10.10.20.0/24 only if router 2 has received a route
to prefix 172.24.19.0/24 from router 3.
Alternatively, if the route to prefix 172.24.20.0 has been installed in the BGP routing table
on router 2, then router 2 advertises to router 1 the route to prefix 10.10.30.0. In this case,
the route does not have to be learned from router 3.
Chapter 1: Configuring BGP Routing
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