network bridge default MTU -- apparent change

[Joe Conway]

On 03/11/2010 04:20 PM, Joe Conway wrote:
> Interestingly after a reboot:
> 
> --------------------
> # brctl show
> bridge name     bridge id               STP enabled     interfaces
> br0             8000.18a9051f09f0       no              eth0
> 
> # ifconfig
> br0       UP BROADCAST RUNNING MULTICAST  MTU:576  Metric:1
> eth0      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
> lo        UP LOOPBACK RUNNING  MTU:16436  Metric:1
> --------------------
> 
> So what is causing br0 MTU == 576 here?

Following up, I tested on a Fedora 12 VM that was pretty much a clean
install from 2 months or so ago. First I changed the network settings to
match my host machine, and rebooted the VM. This is what I saw:

----------------
# ifconfig
br0       UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
eth0      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
lo        UP LOOPBACK RUNNING  MTU:16436  Metric:1
----------------

Looks good so far.

Next I did a "yum update" and rebooted. Keep in mind that this is in a
VM, so I am not running libvirt, qemu, kvm, etc. at all. Now I see:

----------------
# ifconfig
br0       UP BROADCAST RUNNING MULTICAST  MTU:576  Metric:1
eth0      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
lo        UP LOOPBACK RUNNING  MTU:16436  Metric:1
----------------

Here you can see br0 MTU = 576 even though eth0 MTU is 1500. So the
issue seems to be generic with some recent package update.

Unfortunately since the VM had been idle for a long while, there were
hundreds of packages updated.

Anyone have any idea what package would determine the default MTU for a
bridged network device?

Thanks,

Joe

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