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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