IP Routing Policy 259and the matching objects are attributes of routing information. The relationship ofif-match clauses for a node uses a series of Boolean “AND” statements. As a result,a match is found unless all the matching conditions specified by the if-matchclauses are satisfied. The apply clause specifies the actions that are performed afterthe node match test concerning the attribute settings of the route information.The comparisons of different nodes in a route policy uses a Boolean “OR” statement.The system examines the nodes in the route policy in sequence. Once the route ispermitted by a single node in the route-policy, the route passes the matching test ofthe route policy without attempting the test of the next node.ACLThe access control list (ACL) used by the route policy can be divided into three types:advanced ACL, basic ACL and interface ACL.A basic ACL is usually used for routing information filtering. When the user definesthe ACL, the user must define the range of an IP address or subnet for the destinationnetwork segment address, or the next-hop address of the routing information. If anadvanced ACL is used, perform the matching operation by the specified sourceaddress range.For details of ACL configuration, refer to Chapter 7, Using QoS/ACL Commands.IP PrefixThe function of the IP Prefix is similar to that of the ACL, but it is more flexible andeasier for users to understand. When the IP Prefix is applied to routing informationfiltering, its matching objects are the destination address information and the domainof the routing information. In addition, in the IP Prefix, you can specify the gatewayoptions and require it to receive only the routing information distributed by somecertain routers.An IP Prefix is identified by the ip-prefix name. Each IP Prefix can include multiple listitems, and each list item can specify the match range of the network prefix forms,and is identified with an index-number. The index-number designates the matchingcheck sequence in the IP Prefix.During the matching, the router checks list items identified by the sequence-numberin ascending order. Once a single list item meets the condition, it means that it haspassed the ip-prefix filtering and does not enter the testing of the next list item.Configuring an IPRouting PolicyConfiguring a routing policy includes tasks described in the following sections:■ Defining a Route Policy■ Defining If-match Clauses for a Route-policy■ Defining Apply Clauses for a Route Policy■ Importing Routing Information Discovered by Other Routing Protocols■ Defining IP Prefix■ Configuring the Filtering of Received Routes■ Configuring the Filtering of Distributed Routes
