GRE Tunnels and MPLS

Rettke, Brian Brian.Rettke at
Mon Nov 8 13:00:41 CST 2010

This seems to be working now with the 'mls mpls tunnel-recir' command entered. There are some potential downsides, but this should get things up and running until I create the new backup tunnels (GRE over IPSec) on a connected router that is not MPLS-enabled. Thanks!


Brian A . Rettke
Network Engineer, CableONE Internet Services


Message: 6
Date: Thu, 04 Nov 2010 16:49:55 -0400
From: Shimol Shah <shimshah at>
Subject: Re: GRE Tunnels and MPLS
To: nanog at
Message-ID: <4CD31C73.80004 at>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Do you have recir enabled ? If not, good one to enable and check for
status of issue.

"If you do not enable tunnel-MPLS recirculation, the IPv4 and
IPv4-tunneled packets that need to be labeled (for example, the packets
that are encapsulated with an MPLS header) will be corrupted when they
are transmitted from the Cisco 7600 series router."


On 11/4/10 4:00 PM, Rettke, Brian wrote:
> 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 prob
lem with MPLS' interaction with the tunnel. Has anyone encountered something similar?
> Sincerely,
> Brian A . Rettke
> Network Engineer, CableONE Internet Services

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