can I ask mtu question

Niels Bakker niels=nanog at bakker.net
Tue Feb 3 10:09:42 CST 2009


* 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).


	-- Niels.




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