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For some reason MTU still seem to be an occasional issue with trunked connections.

In this case I have a trunk connected to an Enterprise Linux 5 server. I use standard vlan seperation in Linux, and have the vlan interface bridged with virtual interfaces for Xen.

If a guest has the MTU set to the default of 1500, networking breaks. If I configure the guest's MTU to 1496, everything is fine.

As I understand it, Intel Pro/1000 PT is VLAN aware and should let the VLAN tag through, on top of the 1500 byte network package.

What is the trick to make this work? Is there a driver setting or some special way to configure the VLANs?

Additional information:

Dom0 is using the e1000e driver.

Support for jumboframes is disabled on the switches.

Strangely enough, this is only a problem for guests. Dom0 has no MTU problems, despite having connectivity on an alias of a vlan bridge (eg. vlan10:1)

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What does ethtool -d ethX | grep VLAN say? –  Dan Carley Nov 27 '09 at 14:53
    
On the host "ethtool -d eth0 | grep VLAN" reveals nothing, it looks likt the register dump does not contain the phrase "VLAN". On the guest the operation is not supported (as expected). –  Roy Nov 27 '09 at 15:04
    
I should have specified that it was to be run on the host (dom0). If it was, is it using an e1000 driver? –  Dan Carley Nov 27 '09 at 15:06
    
Is something configured to us jumbo frames? –  David Mackintosh Nov 27 '09 at 15:16
1  
No, the e1000e is a driver for a particular strain of Intel NICs. I don't know why the existing e1000 driver couldn't be extended, but I've got machines (which on-board and off-board Intel NICs) that use both the e1000 and e1000e drivers. Very annoying. –  womble Nov 27 '09 at 18:55
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1 Answer

up vote 2 down vote accepted

Things are not always what they seem. It turns out the root cause was the gateway/firewall machine for that VLAN. It was equipped with an older nic supported by the eepro100 driver.

I have now replaced that interface with an SMC 9452TX, and the MTU issue is no more.

Thanks everyone!

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