GRE Tunnels and MPLS

["Rettke, Brian" <Brian.Rettke () cableone biz>]

Beginning work on our implementation of MPLS for the backbone network. I've run into difficulty with our GRE tunnels. 
The GRE Tunnel sits on our co-lo router (a Cisco 7600), and it uses a route-map to push our 10.x modem traffic to our 
DHCP servers. This is because the backbone is not complete and DHCP traffic needs to traverse the internet. What I have 
found is that when I enable basic MPLS on the co-location interfaces that head back to the individual systems, DHCP 
traffic still works, but ICMP and other 10.x traffic dies. There is also an intermittent problem with DHCP when it is 
enabled, where not all DISCOVERS are answered. I've tried everything I can think of, including adjusting MTU and TCP 
MSS. It only seems to impact when the co-location router has a GRE tunnel on one buffer, which it terminates, and then 
it has to encapsulate traffic with an MPLS tag before sending out of the other buffer. Theoretically, it should work, 
but I can't figure out if there is some problem with MPLS' interaction with the tunnel. Has anyone encountered 
something similar?

Sincerely,

Brian A . Rettke
RHCT, CCDP, CCNP, CCIP
Network Engineer, CableONE Internet Services