network bridge default MTU -- apparent change

Joe Conway mail at joeconway.com
Thu Mar 11 22:14:46 UTC 2010


In the last few days I've noticed network connectivity issues from
multiple virtual machines (fedora, centos, winxp) running on a fedora 12
host. What seemed odd was that I could ping by host name, showing that
both the basic network functionality as well as DNS was working. What
was failing was browser access to any site outside my own subnet.

I'm reasonably sure the issue is the MTU setting for my host bridge
(br0) interface. It currently shows:

# ifconfig
br0   [...]
      UP BROADCAST RUNNING PROMISC MULTICAST  MTU:576  Metric:1
      [...]

I would have expected MTU:1500. In fact most of the examples I've found
show other people with br0 having MTU=1500.

My short term workaround has been to manually set MTU to 576 in each of
my VMs. This works, but I'm wondering:

1) Have others seen this?
2) Is there any way to manually increase MTU for the bridge interface?

WRT #2, I tried:

    ifconfig br0 mtu 1500

and get this error:

    SIOCSIFMTU: Invalid argument

I also tried adding MTU=1500 to ifcfg-br0. No joy.

Any ideas?

Thanks,

Joe

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