Hour 475: GRE recursive routing

It’s been almost a year since I have been out of the military and I thought it would be a good idea to dedicate a blog to them. More specifically I want to do a technical blog on a type of problem I sometimes encountered while working there; GRE recursive routing. Any enterprise that runs tunneling protocols like GRE/IPSEC or VTI’s will most likely encounter these type of problems at one point or another.

First, I want to separate this post in two cases; Case A and Case B. They are different scenarios but their root causes are both the same.

Case A: Recursive Routing due to less specific route

Topo_VPN

In case A, we have a topology where R2 and R3 are VPN devices and R1 and R4 are routers running a GRE tunnel between each other sharing routing information through an IGP. Continue reading