MTU mismatch on one link

Tom Taylor tom.taylor.stds at
Fri Aug 31 15:16:21 UTC 2012

My question was actually prompted by an issue that comes up with 
multicast routing, where either the underlying RIB is static or the 
unicast routing protocol managed to operate successfully. In any event, 
in some cases the PIM messages get large enough to be lost at layer 2.

One proposal before the PIM Working Group was to negotiate MTU as part 
of the PIM Hello exchange, but this was considered overkill for a 
debugging-type problem. As some responses have pointed out, OSPF does at 
least detect the situation.

On 31/08/2012 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.

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