say I have site A,B,C,D. connection A and B is my primary link, connection C and D is my secondary. Im running OSPF. I created GRE tunnels end-to-end. If I shut my primary connection Im expecting to route my traffic to secondary. I saw traffic going to secondary, however, my GRE tunnels went down. Why? Do I need to increase the cost of my primary even though I already shuts the link?
In addition to the request from GNT to see the configuration I think it would be helpful to see the output of show ip route when the tunnels are up (and perhaps also when you have the primary shut down).
I suspect that the issue is that with the primary shut down the router does not have a viable path to the tunnel destination. In IOS a GRE tunnel will be up if the router has a viable route to the tunnel destination and will be down if the router does not have a viable route to the tunnel destination.
This does not necessarily mean that GRE depends on end to end connectivity. The GRE tunnel will come up if the router has a route that it believes will get to the destination. There might be a problem in the network which prevents end to end connectivity but the tunnel will be up if the router has a route to the destination. If you want to check end to end connectivity Cisco has introduced a feature of GRE keepalives which is an optional feature in configuring GRE. If you configure GRE keepalives it will check for end to end connectivity and the GRE tunnel will go protocol down if it does not have end to end connectivity.
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