MTU mismatch on one link

Scott Helms khelms at
Fri Aug 31 15:21:56 UTC 2012

If you don't exceed the MTU ever then it shouldn't matter, but unless 
the MTUs are above what the protocol can handle on a specific link you 
probably will.  Most commonly this happens on DSL links using PPPoE (the 
MTU needs to be at 1492 for the overhead) and it causes all kinds of odd 
behavior for clients.

On 8/31/2012 10:42 AM, Andrew K. wrote:
> Besides routing protocol convergence is there any service issues with 
> running mismatched MTU?  Assuming the packet flow does not exceed the 
> smallest MTU value.
> On 8/31/2012 10:28 AM, Dan White wrote:
>> On 08/31/12 09:30 -0400, Tom Taylor wrote:
>>> Has anyone run into a situation where the MTU at one end of a link 
>>> was configured differently from the MTU at the other end? How did 
>>> you catch it?
>>> In general, do you see any need for a debugging tool to be 
>>> standardized to find such mismatches?
>> Performing a ping with a large packet size '-s', and/or with packet
>> fragmentation turned off '-M do' have been our primary tools for finding
>> MTU (layer 2 and layer 3) mismatches.

Scott Helms
Vice President of Technology
(678) 507-5000

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