recognize all routes within the company, so the following must be configured: Redistribution must be configured on the boundary router. Figure displays how router B is configured to accomplish the required redistribution. RIP is redistributed under the OSPF process. In this example, the metric is set under the redistribute command. Other options include specifying a default metric or accepting the OSPF default metric of 20. The default-metric command assigns a seed metric to all routes redistributed into OSPF from any origin. If a metric value is configured under a specific redistribute command, this value overrides the default metric value. A value of 300 is selected because it is a worse metric than any of the native OSPF routes. Under the RIP process, routes are redistributed in from OSPF process number 1. These routes are redistributed into RIP with a metric of 5. A value of 5 is chosen because it is higher than any metric in the RIP network.
Content 5.2 Configuring and Verifying Router Redistribution 5.2.12 Verifying Route Redistribution Example Figure shows the routing tables of all three routers after redistribution is completed. As you can see, the goal has been accomplished: All routers now have routes to all remote subnets. There is complete reachability within the entire network. Routers A and C now have many more routes to keep track of than before. Each router is also affected by topology changes in the routing domain of the other router. Depending on network requirements, you can increase efficiency by summarizing the routes before redistributing them. Remember that route summarization hides information. If routers in the other autonomous systems are required to track topology changes within the network, route summarization should not be performed, because it hides information that the routers need. A more typical case is that the routers need to recognize topology changes only within their own routing domains. In this case, performing route summarization is appropriate. If routes are summarized before redistribution, the routing tables of each router are significantly smaller. Figure displays an example of the routing table after summarization. Router B benefits the most. It now has only four routes to keep track of instead of nine. Router A has five routes instead of eight, and Router C has six routes to keep track of instead of eight. These commands are used to summarize routes for each protocol: RouterA(config)# interface s0
RouterA(config-if)# ip summary-address rip 10.0.0.0 255.252.0.0 Note
This summary includes 10.0.0.0, which is acceptable in this case because it is directly connected with a longer mask. RouterC(config)# router ospf 1
RouterC(config-router)# area 1 range 10.8.0.0 255.252.0.0 To summarize external routes at ASBRs, the command summary-address should be used.
Content 5.2 Configuring and Verifying Router Redistribution 5.2.13 Administrative Distance Problems with Redistribution The following example describes a network using multiple routing protocols. These examples show how a problem can occur and how to identify it. Figure illustrates a network with RIP and OSPF routing domains. OSPF is more believable than RIP, because OSPF has an administrative distance of 110, and RIP has an administrative distance of 120. If, for example, the boundary router (P3R1 or P3R2) learns about network 10.3.3.0 via RIPv2 and also via OSPF, the OSPF route is used and inserted into the routing table, because OSPF has a lower administrative distance than RIPv2, even though the path via OSPF might be the longer (worse) path. Figure illustrates the configurations for routers P3R1 and P3R2. These configurations redistribute RIP into OSPF and OSPF into RIP on both routers. The redistribution into OSPF sets a default OSPF metric of 10,000 to make these routes less preferred than native OSPF routes and protect against route feedback. The redistribute statement also sets the metric type to E1 so that the route metrics continue to accrue, and the router redistributes subnet information. The redistribution into RIP sets a default RIP metric of 5 to also protect against route feedback. Figure displays the routing table on the P3R2 router after redistribution has occurred. The P3R2 router learned RIP and OSPF routes but lists only OSPF routes in the routing table. The first edge router to set up redistribution has a normal routing table and retains the RIP routes. The second edge router chooses the OSPF routes over its RIP routes. The paths to the internal RIP routes are shown as going through the core because of the dual mutual redistribution points. OSPF is informed about the RIP routes via redistribution. OSPF then advertises the RIP routes via OSPF routes to its neighboring router. The neighbor router is also informed about the same routes via RIP. However, OSPF has a better administrative distance than RIP, so the RIP routes are not put into the routing table. OSPF was configured on the P3R1 router first, and P3R2 then received information about the internal (native RIP) routes from both OSPF and RIP. It prefers the OSPF routes because OSPF has a lower administrative distance. Therefore, none of the RIP routes appear in the table. Refer back to the topology diagram to trace some of the routes. The redistribution has resulted in suboptimal paths to many of the networks. For instance, 10.200.200.34 is a loopback interface on router P3R4. P3R4 is directly attached to P3R2. However, the OSPF path to that loopback interface goes through P3R1, then P3R3, then P3R4 before it reaches its destination. The OSPF path taken is actually a longer (worse) path than the more direct RIP path. One of the boundary routers (P3R2 in this example) selected the poor paths because OSPF has a better administrative distance than RIP. You can change the administrative distance of the redistributed RIP routes to ensure that the boundary routers select the native RIP routes, as illustrated in the figure.
Content 5.2 Configuring and Verifying Router Redistribution 5.2.14 Administrative Distance Solution with Redistribution There are a number of ways to correct path selection problems in a redistribution environment. This example shows one possible way. One of the boundary routers (P3R2 in this example) selected the poor paths because OSPF has a better administrative distance than RIP. You can change the administrative distance of the redistributed RIP routes to ensure that the boundary routers select the native RIP routes, as illustrated in the Figure . In Figure , the distance command modifies the administrative distance of the OSPF routes to the networks that match access control list (ACL) 64. Specifically, the distance 125 0.0.0.0 255.255.255.255 64 command assigns the administrative distance of 125 to all routes specified in ACL 64. In this scenario, ACL 64 is used to match all the native RIP routes. The access-list 64 permit 10.3.1.0 command configures a standard ACL to permit