3Com Switch 7750 Configuration Guide

378 CHAPTER 37: IP ROUTING POLICY CONFIGURATION
destination network segment addresses or next-hop addresses of routing
information. If an advanced ACL is used, the specified range of source addresses
will be used for matching.
ip-prefix
ip-prefix plays a role similar to ACL. But it is more flexible than ACL and easier to
understand. When ip-prefix is applied to filtering routing information, its matching
object is the destination address information field of routing information.
An ip-prefix is identified by its ip-prefix name. Each ip-prefix can include multiple
items, and each item, identified by an index-number, can independently specify
the match range in network prefix form. An index-number specifies the matching
sequence in the ip-prefix.
During the matching, the router checks items identified by index-number in
ascending order. Once an item is met, the ip-prefix filtering is passed and no other
item will be checked.
as-path
as-path is an access control list of autonomous system path. It is only used in BGP
to define the matching conditions about AS path. An as-path contains a series of
AS paths which are the records of routing information passed paths during BGP
routing information exchange.
community-list
community-list is only used to define the matching conditions about community
attributes in BGP. A BGP routing information packet contains a community
attribute field used to identify a community.
Applications of Routing
Policy
The following are main applications of routing policy:
■ When a routing protocol advertises or receives routing information, it adopts
routing policy to filter the routing information, so as to receive or advertise only
the routing information meeting given conditions.
■ When a routing protocol imports the routes discovered by other protocols into
itself, it adopts routing policy to import only those routes meeting given
conditions.
In addition, routing policy can also be used to change some route attributes.
IP Routing Policy
Configuration
The configuration of routing policy includes the configuration of filters and the
application of routing policy.
1 You can configure the following filters:
■ Route-policy
■ ACL
■ IP prefix list
■ AS path list
■ Community attribute list