[Beowulf] Mixing different MTU settings on the same LAN?

Bogdan Costescu Bogdan.Costescu at iwr.uni-heidelberg.de
Mon May 18 13:40:34 EDT 2009


On Mon, 18 May 2009, Carsten Aulbert wrote:

> I've run some early tests and these *seem* to suggest that the 
> IP-layer takes correctly care of this (e.g. tcpdump shows that the 
> maximum header length it 1514 bytes when the link is in use).

This could be true for TCP streams; this could also be true for 
sending UDP datagrams but will not be true for receiving UDP datagrams 
(where the sending host will define the packet size based on its MTU). 
Depending where you run tcpdump, you might not see large packets 
because the hardware might filter them before they reach the stack...

The behaviour of a switch trying to transport packets larger than the 
documented MTU is not defined - it can still transport the packets up 
to some higher size (I've seen some switches specified for normal 
frames that were able to transfer up to ~2K), it can drop the packet 
or it can truncate the packet.

Hosts with different MTUs can in theory be connected physically to the 
same network if they don't need to communicate between them. For a 
cluster though this probably doesn't make much sense as there will 
probably be some common point, like a file server, a queue manager, 
etc.

-- 
Bogdan Costescu

IWR, University of Heidelberg, INF 368, D-69120 Heidelberg, Germany
Phone: +49 6221 54 8240, Fax: +49 6221 54 8850
E-mail: bogdan.costescu at iwr.uni-heidelberg.de
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