can I ask mtu question

Tue Feb 3 16:42:19 UTC 2009

Niels Bakker wrote:
> * sam_mailinglists at spacething.org (Sam Stickland) [Tue 03 Feb 2009, 
> 13:04 CET]:
>> For what it's worth, TCP will negiogate MSS and will work with 
>> mismatched MTU in a single LAN segment.
>
> No
>
> Machine 1 -- switch with 1500 byte MTU -- switch with smaller MTU -- 
> switch with 1500 byte MTU -- machine 2
>
> Same situation as when you have IP routers with smaller MTUs in the 
> path that also do not send ICMP Fragmentation Needed errors (or those 
> are dropped on the way to you)
>
> If you configure one of those machines with an MTU equal to or smaller 
> than the smallest MTU in the path then yes TCP (assuming MSS option is 
> used) won't send packets that happen to be too big, but again, same 
> story as for routers vs on a LAN.  The problem isn't that machine 1 
> and 2 in the above example disagree on MTU, the problem is that 
> equipment in the path disagrees on the MTU and cannot (in the case of 
> switches) send notifications of such, or those will not arrive (in the 
> case of stupid firewall admins in control of networks).
Sorry, I should had clarified. I meant mismatched host MTUs within a 
jumbo MTU supporting L2 domain.

Sam