378 CHAPTER 37: IP ROUTING P OLICY CONFIGURATIONdestination network segment addresses or next-hop addresses of routinginformation. If an advanced ACL is used, the specified range of source addresseswill be used for matching.ip-prefixip-prefix plays a role similar to ACL. But it is more flexible than ACL and easier tounderstand. When ip-prefix is applied to filtering routing information, its matchingobject is the destination address information field of routing information.An ip-prefix is identified by its ip-prefix name. Each ip-prefix can include multipleitems, and each item, identified by an index-number, can independently specifythe match range in network prefix form. An index-number specifies the matchingsequence in the ip-prefix.During the matching, the router checks items identified by index-number inascending order. Once an item is met, the ip-prefix filtering is passed and no otheritem will be checked.as-pathas-path is an access control list of autonomous system path. It is only used in BGPto define the matching conditions about AS path. An as-path contains a series ofAS paths which are the records of routing information passed paths during BGProuting information exchange.community-listcommunity-list is only used to define the matching conditions about communityattributes in BGP. A BGP routing information packet contains a communityattribute field used to identify a community.Applications of RoutingPolicyThe following are main applications of routing policy:■ When a routing protocol advertises or receives routing information, it adoptsrouting policy to filter the routing information, so as to receive or advertise onlythe routing information meeting given conditions.■ When a routing protocol imports the routes discovered by other protocols intoitself, it adopts routing policy to import only those routes meeting givenconditions.In addition, routing policy can also be used to change some route attributes.IP Routing PolicyConfigurationThe configuration of routing policy includes the configuration of filters and theapplication of routing policy.1 You can configure the following filters:■ Route-policy■ ACL■ IP prefix list■ AS path list■ Community attribute list
