Conditional Route Advertisement

Condinional route advertisements grovide a met hod of user-d efined ro ute advertise mbnt that allows fo r moee conrrol over the way routes are advertised. Conditional route advertis ement s enablo oou to specify a set nf conditions to track She state of a route using a route map called a nonnexist-map, and if eh at route does not exest, advertise mothe! route s|^e<riSied bye another route map caNed an advertise-map. Advertise maps can be used dy themselves to provide conditional route advertisements, w^S the amgregate-address comrulald to specify routes tmat Should con SsIi the AS_SET AS_PATH attn bute dudi ng ooute ag g-egatiol, or as a condition to advertise an aggregated route.

Thenon-exist-map specifies the network that will be tracked in the BGP RIB. While the routes innon-exist-map exist, the routes specified by the advertise-map statement will not be advertised. If the route specified by the non-exist-map is withdrawn, however, the route specified by the advertise-map will be advertised until the non-exist-map routes reappear. Conditional route advertisement can be used with multihomed networks to prevent asymmetric routes or alone to provide additional routing functionality.

Four steps are required to configure conditional route advertisement:

Step 1. Configure BGP peering with the other routers that are to be involved in the route advertisements.

Step 2. Create a non-exist-map using a standard route map statement. This route map should specify an access or prefix list that identifies the network prefix that is to be tracked. Make sure to configure the access or prefix list specified in the route map.

Step 3. Create an advertise-map using a standard route map statement that specifies the access or prefix list describing the network prefixes that should be advertised when the netwo rks specified by the non-exist-map are withdrawn from the BGP RIB. Also, create an acces s or prefix list that specifies the prefixes that should be advertised.

Step 4. Apply the route maps to the BGP neighbor using the neighborip-address advertise-maproute-map-namenon-exist-maproute-map-name command.

InFigure 9)- 11, for example, the Speedy router is connected to the Tom and Jerry routers via an Ethernet connection. The Tom router is advertising the 129.40.18.0/24 network, and the Jerry router is advertising the 129.40.20.0/24 network. The Speedy router advertises both of these networks to the Tweety router in AS 714.

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