Re: network bridge default MTU -- apparent change

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On 03/11/2010 02:31 PM, Rick Stevens wrote:
> 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.

# brctl show
bridge name     bridge id               STP enabled     interfaces
br0             8000.18a9051f09f0       no              eth0
                                                        vnet0
virbr0          8000.000000000000       yes

This is pretty much as I expected based on research...

> 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.

# ifconfig -a
br0       UP BROADCAST RUNNING PROMISC MULTICAST  MTU:576  Metric:1
eth0      UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
lo        UP LOOPBACK RUNNING  MTU:16436  Metric:1
sit0      NOARP  MTU:1480  Metric:1
virbr0    UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
vnet0     UP BROADCAST RUNNING MULTICAST  MTU:576  Metric:1
wlan0     BROADCAST MULTICAST  MTU:1500  Metric:1

I had failed to notice vnet0 before -- that seems to be the culprit! Now
the next question is why did it change (or get added?), and how do I fix
it? Thanks for getting me to the next step!

Joe

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