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BGP: Configuring Route Maps
Page 34 | Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes
Particular mention, though, has to be made of the case where the match clause specifies
prefix list or ACL as the match criterion, and the route map is being applied to outgoing
route updates.
The intention of such an entry would be that the attribute values specified in the set clauses
be applied to only those routes that are contained in the prefix list or ACL specified in the
match clause.
What happens when there is an outgoing update message that contains several prefixes,
some of which are selected by the match clause and some of which are not? Given that an
update packet contains just one set of attributes, it is not possible to say “change some of the
attributes in the packet, but make them apply to only some of the prefixes contained in the
packet”. The attributes in an update packet must apply to all the prefixes in the packet.
The only solution is to break up the update packet. The switch creates one packet that
contains the original attribute values and the prefixes that were not specified in the match
clause of the route map entry. Then the switch creates another packet that contains those
prefixes of the original packet which did match the prefix list or ACL in the match clause, but
with modified attribute values.
The following diagram illustrates the BGP process of splitting an outgoing update:
BGP process of splitting an outgoing update when a filter changes theAS attribute for some of the prefixes in the update
Update from Peer 1
ASPathList=4,3,2,1
192.168.1.0/24
192.168.2.0/24
172.30.0.0/16
172.31.0.0/16
Update 1 to Peer 2
ASPathList = 5,8,4,3,2,1
192.168.1.0/24
192.168.2.0/24
Add peer Update to IP table
Router generates a new Update, from
the BGP route table, to send to another
peer
Take internally generated Update and
add theAS“8” for matches of
192.168.0.0/16
Router (AS 5)
Update 2 to Peer 2
ASPathList = 5,4,3,2,1
172.30.0.0/16
172.31.0.0/16