Ethernet Circuit MTU

Posted on 2011-05-10
Medium Priority
Last Modified: 2012-05-11

We're experiencing some wierdness with our network - a brief overview;

We have 2 sites (within 50 miles) using the same ISP, same firewall hardware.  MTU's on each device (switch, firewall etc) is set to 1500 but when we perform ping IP -f -l MTU we get fragmented packets for any MTU value over 1272.

Also, all sites we communicate with (32 sites worldwide) we're gettig the same response with over MTU 1272.

Is this normal? Should we be shouting at the ISP?

Clearly if the MTU is low, then we're getting less for our buck..?

Question by:jasonhamlett

Accepted Solution

Michael Knight earned 2000 total points
ID: 35730687
It's probably just the route. I mean every node that the transmission is going through is going to have and modify the MTU where possible according to the Path MTU Discovery 'protocol'. Meaning the entire route/path is going to be 'discovered' and adjusted to lowest MTU for any device that the packet will encounter. Forcing a 1500 would then of course become fragmented.
1272 does seem low, but it's most likely a failsafe low. If you wanted to control the MTU outside your network you'd have to control every hop.

Author Closing Comment

ID: 36559705
got me thinking in the right direction...

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