Most networks you encounter will probably only run a single routing protocol like OSPF or EIGRP. Maybe you find some old small networks that are still running RIP that need migration to OSPF or EIGRP. What if you have a company that is running OSPF and you just bought another company and their network is running EIGRP?
It’s possible that we have multiple routing protocols on our network and we’ll need some method to exchange routing information between the different protocols. This is called redistribution. We’ll look into some of the issues that we encounter. What are we going to do with our metrics? OSPF uses cost and EIGRP uses K-values and they are not compatible with each other….RIP uses hop count.
Redistribution also adds another problem. If you “import” routing information from one routing protocol into another it’s possible to create routing loops.
If you don’t feel 100% confident about your knowledge on OSPF and EIGRP then I suggest you stop reading now and read more about OSPF / EIGRP or do some labs. One routing protocol can be difficult but when you mix a couple of them the fun really starts…
Having said that, let’s take a look at a possible redistribution scenario:
Look at the topology picture above. We have routers running EIGRP in AS 1 with the 10.0.0.0 /8 network. OSPF has multiple areas and we have 184.108.40.206 /8 there. At the bottom there are two RIP routers in the 220.127.116.11 /8 network. If we want to have full connectivity in this network we’ll have to do some redistribution.
Redistribution is not just for between routing protocols, we have multiple options:
- Between routing protocols (RIP, OSPF, EIGRP, BGP).
- Static routes can be redistributed into a routing protocol.
- Directly connected routes can be redistributed into a routing protocol.
Normally you use the network command to advertise directly connected routes into your routing protocol. You can also use the redistribute connected command which will redistribute it into the routing protocol. Let’s take a look at some real routers:
In the topology picture above I have three routers. R1 is running EIGRP and R3 is running RIP. R2 is in the middle and is running EIGRP and RIP. If we want to do redistribution we’ll have to do it on R2. Let’s take a look shall we?
R1(config)#router eigrp 12 R1(config-router)#no auto-summary R1(config-router)#network 192.168.12.0 R1(config-router)#network 18.104.22.168 0.0.0.255
R2(config)#router eigrp 12 R2(config-router)#no auto-summary R2(config-router)#network 192.168.12.0 R2(config-router)#exit R2(config)#router rip R2(config-router)#version 2 R2(config-router)#no auto-summary R2(config-router)#network 192.168.23.0
R3(config)#router rip R3(config-router)#version 2 R3(config-router)#no auto-summary R3(config-router)#network 192.168.23.0 R3(config-router)#network 22.214.171.124
Here are the router configurations, nothing special…I only advertised the links to get EIGRP and RIP up and running.
R1#show ip route Gateway of last resort is not set C 192.168.12.0/24 is directly connected, FastEthernet0/0 126.96.36.199/24 is subnetted, 1 subnets C 188.8.131.52 is directly connected, Loopback0
R2#show ip route Gateway of last resort is not set C 192.168.12.0/24 is directly connected, FastEthernet0/0 184.108.40.206/24 is subnetted, 1 subnets D 220.127.116.11 [90/156160] via 192.168.12.1, 00:05:01, FastEthernet0/0 R 18.104.22.168/8 [120/1] via 192.168.23.3, 00:00:12, FastEthernet1/0 C 192.168.23.0/24 is directly connected, FastEthernet1/0
R3#show ip route Gateway of last resort is not set 22.214.171.124/24 is subnetted, 1 subnets C 126.96.36.199 is directly connected, Loopback0 C 192.168.23.0/24 is directly connected, FastEthernet0/0
Here are the routing table of all three routers after configuring RIP and EIGRP. You can see R2 has learned the loopback interfaces of R3 and R1. R1 and R3 don’t have anything in their routing table because R2 is not advertising anything. As you can see redistribution is not done automatically.
Before I show you the redistribution configurations there are two things you should be aware of: