GRE Tunnels and MPLS

Rettke, Brian Brian.Rettke at
Tue Nov 9 19:04:16 CST 2010

It appears that about half of our web traffic is now being dropped, so problems continue. I'll have to double check MTU and TCP adjust-mss settings, but other than that I have no idea. I've heard that there are some issues with MPLS using some of the 67xx linecards, and apparently this is going to be a huge problem for us.
From: Shimol Shah [shimshah at]
Sent: Monday, November 08, 2010 1:19 PM
To: nanog at
Subject: Re: GRE Tunnels and MPLS

Good deal. Sounds like a plan.


On 11/8/10 2:00 PM, Rettke, Brian wrote:
> 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!
> Sincerely,
> 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."
> Shimol
> 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 pro
> 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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