I have router a with a gre tunnel (tunn 1)to router b. Gig 0/0/0 to Gig 0/0/0
Do those two interfaces share a link and in the same subnet?
C is connected to B on gig 0/0/1.
Ditto question?
If answers, above, are yes, then why tunnels? A learning lab? Could you explain purpose and topology choice for these tunnels?
That aside, you can "multiplex" the same interface IP for multiple tunnels. (The one device, I recall it was recommended not to do this on was the 6500. For it, each tunnel was recommended to have its own source interface, which could be accomplished by using loopbacks. [I believe this was to insure optimal hardware processing of each tunnel's traffic.])
Could the second tunnel use the first tunnel's IPs? I'm unsure that it's impossible. You would have to be very careful with routing loops caused by faulty recursive routing, which often happens to those new to tunneling even with a single tunnel.
M02@rt37 mentions
Be sure Router C has got a static route back to Router A's source IP through Router B for the tunnel to come up successfuly.
He's 100% correct, router C needs to know how to reach router A's tunnel destination, but the converse is true too, i.e. A=>C. Further this applies to all transit L3 hops, like router B. But although static route statements are generally used, how routing is structured is an "it depends". For example if both routers A and C had a default static to router B, that might work in your case (cannot be sure as you haven't fully described your topology).
