Chapter 50
| IP Routing Commands
Policy-based Routing for BGP
– 1135 –
Example
Console#show ip prefix-list summary rd
ip prefix-list rd:
count: 1, range entries: 0, sequences: 5 - 5
Console#
Policy-based Routing for BGP
This section describes commands used to configure policy-based routing (PBR)
maps for Border Gateway Protocol (BGP).
Policy-based routing is performed before regular routing. PBR inspects traffic on
the interface where the policy is applied and then, based on the policy, makes
some decision. First, the traffic is “matched” according to the policy. Second, for
each match, there is something “set.” What is set could be that the traffic matches
must exit out a different interface, or the traffic could be given a higher priority, or it
could choose to just drop that traffic.
Matching of the traffic is usually done with an ACL (access-control list) that is
referenced by a route-map. In the route-map, if there is a “match” for the traffic
defined in that ACL, then a “set” defines what the administrator wants to happen to
that traffic (prioritize it, route it differently, drop it, or other actions). Policies can be
based on IP address, port numbers, protocols, or size of packets.
If matching criteria is found and the specified action is to permit the packet, then it
will be forwarded to the next hop based on policy-based routing. If the action is to
deny the packet, normal unicast routing is used to determine the packet’s next hop,
instead of using policy-based routing. If no matching criteria are found in the route
map, normal unicast routing is used to determine the packet’s next hop. Although
route redistribution is protocol-independent, some of the route-map match and set
commands defined in this section are specific to BGP.
Like matches in the same route map subblock are filtered with “or” semantics. If any
one match clause is matched in the entire route map subblock, this match is
treated as a successful match. Dissimilar match clauses are filtered with “and”
semantics. If the first set of conditions is not met, the second match clause is
filtered. This process continues until a match occurs or there are no more match
clauses.
A route map can have several sequences. A route that does not match at least one
match command defined in a route-map will be ignored; that is, the route will not
be advertised for outbound route maps nor accepted for inbound route maps.
Table 29: Policy-based Routing Configuration Commands
Command Function Mode
route-map Enters route-map configuration mode, allowing route
maps to be created or modified
GC
call Jumps to another route map after match and set
commands are executed
RM
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