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Cisco 9951/9971, VPN, MTU, Fragmentation: how does it work?


We have a set of 9951/9971 video phones running recent firmware versions (CUCM 9.1(2)).

When two of these phones call each other one party sees garbled video. We've been able to determine that one phone is not receiving all packets from the remote phone (one-way garbled video).

As these packets need to travel across a VPN (DMVPN, MTU 1400) video packets are being fragmented and get lost (somewhere), we determined that.

My question now is; how is the phone supposed to handle VPN's, a lower MTU, fragmentation?

Some facts:

- The local phone ethernet MTU is 1500 bytes

- The phone sents packets with the "Don't Fragment bit" unset (which allows the network to fragment video packets).

- The phone sents video packets up to 1426 bytes in length (as seen by a packet capture on the VPN router on the ethernet side).

- The network fragments the packets

- The phone reassembles packets

I know my problem probably is a network issue but I'd like to know in more detail what (if anything) the phone does to prevent fragmentation or handle fragmentation.


Erik Tamminga

New Member

I've had a TAC case SR

I've had a TAC case SR 629554885 open for this issue for about 2 months without resolution.  Specifically In our case I have an ASA at the remote end which drops the packet fragments as they appear as some type of attack due to the way they are fragmented.  This causes the phones to receive garbled video and eventually crash and reboot.  Did you ever find a workaround?


Hi,We also have a case open


We also have a case open (for months now). SR 627971355 We think we loose the packets at our head-end DMVPN router. Also maybe due to inspection rules/policies. So far Cisco hasn't found the root-cause or didn't come up with a solution.

Please let me know how you proceeed. I'd like to learn from this!



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