Hi Lionel,

It seems to me that the problem is that your SDSL interface mtu may be something like 1500 bytes.

The size of the original packets + the size of the GRE encapsulation (24 bytes) exceeds this 1500 bytes. This is why the packets are fragmented so the router can transmit them over the physical interface with a smaller mtu.

The solutions may be different, but all of them lead to the same result:

The final packet size (original packet size + GRE encapsulation (24 bytes)) should not exceed the SDSL interface mtu.

1. Manually limit the packet size on your hosts. For this, you have to make a little calculation.

Packet size = SDSL interface mtu - 24 bytes

2. Enable path-mtu discovery on the hosts and set the DF-bit (Don't Fragment) to 1 on all packets. The path-mtu discovery process will take care of setting the packet size that can be transmitted over the path that includes the GRE tunnel.

3. If your traffic is tcp traffic only: configure the maximum tcp segment size on the router tunnel interfaces with the "ip tcp adjust-mss" command.

Again you have to do some calculation:

The tcp maximum segment size is calculated as follows:

TCP MSS = SDSL interface mtu - 24 bytes - 40 bytes

24 bytes = GRE encapsulation

40 bytes = the size of the original ip header + tcp header.

Note:

If PPPoE is configured on the SDSL interface then you should also take into account the size of PPPoE encapsulation.

Cheers:

Istvan

To clarify the information both Vishwa and Istvan provided, if possible, you want both PMTU working and "ip tcp adjust-mss". The former informs the host to reduce its MTU if its too large, the latter avoids the overhead involved and too large MTU (initially) for TCP.

Also, the "ip tcp adjust-mss", if I recall correctly, is a later IOS feature. I.e. It might not be found on your 2621MXs without an IOS version upgrade.