[c-nsp] MTU size, fragmentation and drops

Mikael Abrahamsson swmike at swm.pp.se
Fri Apr 15 06:35:21 EDT 2016

On Fri, 15 Apr 2016, CiscoNSP List wrote:

> My question is why?  i.e. Why would reducing our PE's Ints MTU size to 
> 1500 "allow" packets above 1500bytes to pass fragmented, but at 9100, 
> they were dropped?

There are two values here, MTU and MRU.

Some devices configured for 1500 MTU are capable of receiving 9100 
unfragmented packet, but will not transmit more than 1500 bytes (thus 
fragmenting before sending).

So most likely, the CE had MTU and MRU set to 1500, and since the 9100 
sized unfragmented packet sent from the PE to the CE was higher than the 
MRU, it was dropped.

If there were some L2 switches in between the PE and CE, then it/they 
might have dropped it.

Mikael Abrahamsson    email: swmike at swm.pp.se

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