network bridge default MTU -- apparent change

[Rick Stevens]

On 03/11/2010 02:14 PM, Joe Conway wrote:
> 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?

"brctl show" will show how all the bridges are built.  Check all 
interfaces under "ifconfig -a" and see if any of the participants in the 
bridge have a small MTU.  IIRC, the smallest MTU will be propagated
to the bridge so it doesn't overrun the least-capable interface.
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- Rick Stevens, Systems Engineer, C2 Hosting          ricks at nerd.com -
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