In a previous post, we considered the need for route redistribution, and we also took a look at some configuration examples. This posts builds on that previous configuration and discusses how we can filter routes using route maps.
Specifically, the previous example performed mutual route redistribution between EIGRP and OSPF, where all routes were redistributed between the two autonomous systems. However, some design scenarios might want us to prevent the redistribution of every single route. One way to do that filtering is to use a route map.
For your reference, here’s the topology we’re working with:
Also, with our current route redistribution configuration, the IP routing table on router R1 looks like this:
Let’s say, for some reason, we don’t want the 192.168.2.0 /24 network redistributed from EIGRP into OSPF. One way to do that filtering is to use a route map that references an access control list (ACL).
First, let’s go to router R2 and create an ACL that matches the network we wish to filter.Notice the use of the keyword permit in the ACL. In this context, the keyword permit means to match, not allow. We’re next going to reference this ACL in a route map, and it’s the route map that will say we want to deny this network from being redistributed. Here’s how we can create that route map:
Notice we have two route-map statements, with sequence numbers of 10 and 20. Just like an ACL, route maps are processed top-down. In this example, we want to deny network 192.168.2.0 /24 with sequence number 10. Then, in order to allow all other traffic to be redistributed, we create a route-map statement with a sequence number of 20. Notice that unlike the previous route-map statement (which contained a keyword of deny), this route-map statement contains a keyword of permit. As a result, without needing to specify a match condition, we match (and will permit) all remaining routes.
Next, let’s apply our route map to the redistribute command in our OSPF routing process on router R2. Currently, the redistribute command for the OSPF routing process looks like this: redistribute eigrp 1 metric-type 1 subnets
What we want to do is reissue that command, while adding on a reference to our newly created route map.
Now, let’s go back over to router R1 and see if the 192.168.2.0 /24 network has disappeared from the IP routing table.
Great news! The 192.168.2.0 /24 route has been successfully filtered.
That’s going to do it for this post, but we’re not finished discussing route redistribution. In an upcoming post, we’ll take a look at how we can troubleshoot route redistribution issues.
Take good care,
Kevin Wallace, CCIEx2 (R/S and Collaboration) #7945