490 CHAPTER 37: R OUTING POLICY CONFIGURATIONAS-pathAS path is only applicable to IPv6 BGP. There is an AS-path field in the IPv6 BGPpacket. An AS path list specifies matching conditions according to the AS-pathfield.Community listCommunity list only applies to IPv6 BGP. The IPv6 BGP packet contains acommunity attribute field to identify a community. A community list specifiesmatching conditions based on the community attribute.Extended community listExtended community list (extcommunity-list) applies to IPv6 BGP only. It is used forRoute-Target extcommunity for VPN.Routing policyA routing policy is used to match against some attributes in given routinginformation and modify the attributes of the information if match conditions aresatisfied. It can reference the above mentioned filters to define its own matchcriteria.A routing policy can comprise multiple nodes, which are in logic OR relationship.Each node is a match unit, and the system compares each node to a packet in theorder of node sequence number. Once a node is matched, the routing policy ispassed and the packet will not go through the next node.Each node comprises a set of if-match and apply clauses. The if-match clausesdefine the match criteria. The matching objects are some attributes of routinginformation. The different if-match clauses on a node is in logical ANDrelationship. Only when the matching conditions specified by all the if-matchclauses on the node are satisfied, can routing information pass the node. Theapply clauses specify the actions to be performed after the node is passed,concerning the attribute settings for routing information.Routing PolicyApplicationA routing policy is applied in two ways:■ When redistributing routes from other routing protocols, a routing protocolaccepts only routes passing the routing policy.■ When receiving or advertising routing information, a routing protocol uses therouting policy to filter routing information.Defining Filtering ListsPrerequisites Before configuring this task, you need to decide on:■ IP-prefix list name■ Matching address range■ Extcommunity list sequence number
