Cisco CCNP / BSCI Certification: Route Redistribution And The Seed Metric

In the first part of this free CCNP / BSCI tutorial, we looked at how leaving one simple word out of our route redistribution configuration – “subnets” – resulted in an incomplete routing table when redistributing routes from RIP to OSPF. (If you missed that part of the tutorial, visit my website’s “Free Tutorials” section.) Today, we’ll look at redistributing OSPF routes into RIP and identify another common redistribution error.We are using a three-router network. R5 is running RIP, R1 is serving as a hub between R5 and R3 and is running RIP and OSPF, and R3 is running OSPF.To begin this lab, we’ll add three loopbacks to R3 and advertise them to R1 via OSPF.R3(config)#int loopback33R3(config-if)#ip address loopback34R3(config-if)#ip address loopback35R3(config-if)#ip address ospf 1R3(config-router)#network area 1R3(config-router)#network area 1R3(config-router)#network area 1R1 sees all three of these routes in its routing table.R1#show ip route ospf34.0.0.0/32 is subnetted, 1 subnetsO IA [110/65] via, 00:00:55, Serial035.0.0.0/32 is subnetted, 1 subnetsO IA [110/65] via, 00:00:45, Serial033.0.0.0/32 is subnetted, 1 subnetsO IA [110/65] via, 00:00:55, Serial0We’ll now redistribute these routes into RIP on R1. Remember the “subnets” option we talked about in the first part of this tutorial? There is no such option when redistributing OSPF routes into RIP, as IOS Help shows us.R1(config)#router ripR1(config-router)#redistribute ospf 1 ?match Redistribution of OSPF routesmetric Metric for redistributed routesroute-map Route map referencevrf VPN Routing/Forwarding InstanceR1(config-router)#redistribute ospf 1The routes have been redistributed into RIP with the redistribute ospf 1 command. (The “1” is the OSPF process number.) Let’s look at R5 and see the results.R5#show ip route ripR5#The routes aren’t there, but we didn’t get a warning from the router that we needed to do anything else. What is the problem?The problem is that RIP requires a seed metric to be specified when redistributing routes into that protocol. A seed metric is a “starter metric” that gives the RIP process a metric it can work with. The OSPF metric of cost is incomprehensible to RIP, since RIP’s sole metric is hop count. We’ve got to give RIP a metric it understands when redistributing routes into that protocol, so let’s go back to R1 and do so.R1(config)#router ripR1(config-router)#no redistribute ospf 1R1(config-router)#redistribute ospf 1 metric 2R5 now sees the routes. Note that the metric contained in the brackets is the seed metric.R5#show ip route rip34.0.0.0/32 is subnetted, 1 subnetsR [120/2] via, 00:00:24, Ethernet035.0.0.0/32 is subnetted, 1 subnetsR [120/2] via, 00:00:24, Ethernet033.0.0.0/32 is subnetted, 1 subnetsR [120/2] via, 00:00:24, Ethernet0If you read the previous tutorial, you may have noticed that we did not specify a seed metric for OSPF. OSPF does not require a seed metric to be set during redistribution. You also noticed that the router did tell us that there might be a problem when we left the “subnets” option out of RIP>OSPF redistribution, but the router didn’t tell us anything about a seed metric when we performed OSPF>RIP redistribution. This is a detail you must know by heart in order to make your route redistribution successful!

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Ccnp, bsci, exam, route, redistribution, ospf, rip, seed, metric, routing, table, subnet