GRE tunnel - output drops on physical interface

Unanswered Question
Jun 13th, 2008

I have multiple circuits into a remote site. One of these circuits is a PTP DS3, and the other is a frac DS3 over MPLS. The MPLS connection is running BGP. I have a GRE tunnel configured over the MPLS circuit between 2 - 3845 routers.

EIGRP is configured in 1 AS and runs across the PTP and the GRE tunnel as well as the remote network. The problem that I have is that as soon as the GRE tunnel is enabled, the physical interface starts dropping packets. These drops only occur at one of the sites and it is consistent. When the tunnel interface is shut down, the drops stop occurring.

Any ideas on what is causing the dropped packets?

I have this problem too.
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merryllem Fri, 06/13/2008 - 13:22

Try configuring the MTU on the tunnel interface on both ends.

"ip mtu 1472"

rmartinjr24 Fri, 06/13/2008 - 16:03

I looked at the frag counters on a show ip traffic, and they were not incrementing. The CPU was also below 10%. I will give that a shot though and see if that resolves the issue.

Thanks

royalblues Fri, 06/13/2008 - 20:58

What IOS version you are running?

The Counters may also increase due to some Cosmetic bug

Narayan

rmartinjr24 Mon, 06/16/2008 - 08:49

Attached are the brief configs and a diagram. Here is what we are trying to do and why…

Site A has an internet connection and is the the default route for site B. Traffic is currently routed over the PTP DS3 where EIGRP is running. We now have connected a backup MPLS provider. This MPLS cloud has other a lot of other sites connected to it, one of which is injecting the default route. The site injecting the default route has the internet connection for all of the other sites into this cloud (except for site a and b).

In order to get the default route sent from site a to site b, we have decided to bring up a GRE tunnel. Now that the GRE tunnel is up, EIGRP can run across this and after adjusting the interface delay, the MPLS path is the preferred path. One additional item is that the same routes are learned via BGP, but I am summarizing at both sides. The EIGRP topo has the more specific route, therefore is in the routing table on the MPLS WAN routers. I am blocking the default route from getting learned via BGP at site B. This route is getting getting redistributed into EIGRP from the core at site A.

Once I bring this tunnel up, drops occur on the serial interface. I have tried adjustinting the MTU, but the same occurred. I actually started to get drops on the tunnel, but nothing compared to the amount on the serial. In addition to setting the MTU to 1476, I have tried tunnel path-mtu-discovery as well.

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