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Old 05-04-2012   #1
Joe Vallender
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Default C40 exceeding MTU size

Has anyone encountered an issue with a Cisco C40 failing video due to TCP packets being rejected by its router due with ICMP Fragmentation needed? Apparently, the packet (1541 bytes) is exceeding the MTU size of 1500 in the router. Other codecs at that same location (MXP's) thru the same router with the video call from the same Codian 8510 do not have the issue.
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Last edited by Joe Vallender; 05-04-2012 at 08:45 AM.
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Old 05-04-2012   #2
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The MXP used to have a default MTU lower than 1500 (it changed a few times so I don't remember the current value) to make room for encryption overhead. There was also a command for changing the MTU size in MXP that I would hope is also in C-series. Have you tried playing with that?

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Old 05-04-2012   #3
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From the name of the parameter on MXP it was only regards the RTP (=udp)
media pacekts:

xConfiguration RTP MTU ?

*h xConfiguration RTP MTU: <400..1400>
It might have been interesting to see the complete trace to see the message
which triggered the ICMP message. As its a feedback on a TCP packet
was it a h323 / sip message or somthing different? Did this message had a DF bit set?

On the C-Series systems it seems to set the whole system MTU (tc5.1):

xConfiguration Network 1 MTU ?

*? xConfiguration Network [1..1] MTU: <576..1500>
You could try to set it to 1400 and see what happens.

But maybe its something strange in addition in your setup, but like I said
with just the screenshot its hard to tell.
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