This document provides you an example configuration on how to overcome recursive routing issue while using GRE tunnels.
It is assumed that reader already has knowledge of routing protocols and is aware of GRE configuration.
Cisco 3700 series Router loaded with IOS 12.4(2)T has been used for this document.
For Initial configuration, these routers are learning all the routes using RIP protocol. If you are interested in seeing initial configuration, please have a look at the files attached.
Now let’s suppose R1 and R3 have networks connected behind them and both router R1 and R3 need to know about all those networks and because of scalability issues that we have with RIP protocol, we need to run some other protocol. Let that protocol be EIGRP. Also, let’s suppose that because of resources limitation we can’t run EIGRP on R2. So you decide to use GRE tunnel between routers R1 and R3.
Here is the configuration for the same:
ip address 188.8.131.52 255.255.255.0
tunnel source Loopback0
tunnel destination 184.108.40.206
ip address 220.127.116.11 255.255.255.0
tunnel source Loopback0
tunnel destination 18.104.22.168
Now as soon as we enable EIGRP and run it on tunnel interface and the loo 0 interface, we get following error:
This happens because the tunnel interface status depends on the reachability to tunnel destination. In our case R1 and R3 find reachability to tunnel destination using RIP. As soon as we enable EIGRP on loo 0 interfaces, the routers find reachability to tunnel destination through tunnel and that’s what causes the recursive loop.
To overcome this situation, an access-list in configured which identifies the IP add of tunnel destination. Then, AD of this route is configured lower than EIGRP. Once this is done router find better path to tunnel destination throught RIP and tunnel comes up (you may have to clear routing table at this point).
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