BGP Conditional Advertisement
BGP, by default, advertises all the best paths in its BGP table to external peers. Sometimes, it might be required that BGP advertises some paths conditionally, meaning, advertise paths only if some other routes are available.
Conditional advertisement has two forms- advertisement of some prefix(es) when some other prefix(es) do not exist and advertisement of some prefix(es) when some other prefix(es) exist. The prefixes to be advertised are defined by advertise-map. The condition is defined by a route-map called non-exist-map for conditions that do not exist or by a route-map called exist-map for conditions that do exist.
The first form of conditional advertisement is configured as follows-
BGP, by default, advertises all the best paths in its BGP table to external peers. Sometimes, it might be required that BGP advertises some paths conditionally, meaning, advertise paths only if some other routes are available.
Conditional advertisement has two forms- advertisement of some prefix(es) when some other prefix(es) do not exist and advertisement of some prefix(es) when some other prefix(es) exist. The prefixes to be advertised are defined by advertise-map. The condition is defined by a route-map called non-exist-map for conditions that do not exist or by a route-map called exist-map for conditions that do exist.
The first form of conditional advertisement is configured as follows-
The map2 route-map tracks the prefix(es) in the local router. If a match is made, the status of the non-exist-map is Withdraw; when no match is made, the status is Advertise.
The map1 route-map defines the prefix(es) that are to be advertised when the status of non-exist-map is Advertise. When the status of the non-exist-map isWithdraw, the prefix(es) in advertise-map are not advertised.
The second form of conditional advertisement is configured as follows-
Again, the map2 route-map tracks the prefix(es) in the local router. If a match is made, the status of the exist-map is Advertise; when no match is made, the status of the exist-map is Withdraw. The function is map1 route-map is same as above.
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