6
IBGP
200
EBGP
20
Unknown route
255
1.2 IP Routing Policy
1.2.1 Introduction to Routing Policy
Some policies have to be applied when the router publishing and receiving routing
messages so to filter routing messages, such as only receiving or publishing routing messages
meets the specified conditions. A routing protocol maybe need redistribute other routing
messages found by other protocols such as OSPF so to increase its own routing knowledge;
when the router redistributing routing messages from other routing protocols there may be only
part of the qualified routing messages is needed, and some properties may have to be
configured to suit this protocol.
To achieve routing policy, first we have to define the characteristics of the routing messages
to be applied with routing policies, namely define a group matching rules. We can configure by
different properties in the routing messages such as destination address, the router address
publishing the routing messages. The matching rules can be previously configured to be applied
in the routing publishing, receiving and distributing policies.
Five filters are provided in ES4624-SFP/ES4626-SFP switch: route-map, acl, as-path,
community-list and ip-prefix for use. We will introduce each filter in following sections:
1. route-map
For matching certain properties of the specified routing information and setting some routing
properties when the conditions are fulfilled.
Route-map is for controlling and changing the routing messages while also controlling the
redistribution among routes. A route-map consists of a series of match and set commands in
which the match command specifies the conditions required matching, and the set command
specifies the actions to be taken when matches. The route-map is also for controlling route
publishing among different route process. It can also used on policy routing which select
different routes for the messages other than the shortest route.
A group matches and set clauses make up a node. A route-map may consist of several
nodes each of which is a unit for matching test. We match among nodes with by
sequence-number. Match clauses define matching rules. The matching objects are some
properties of routing messages. Different match clause in the same node is “and” relation
logically, which means the matching test of a node, will not be passed until conditions in its entire
match clause are matched. Set clause specifies actions, namely configure some properties of