Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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)

share|improve this question
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
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
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!

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.