On 03/11/2010 02:52 PM, Joe Conway wrote:
> On 03/11/2010 02:31 PM, Rick Stevens wrote:
>> 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!
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?
Now, if I start up a VM:
--------------------
# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.18a9051f09f0 no eth0
vnet0
# 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
vnet0 UP BROADCAST RUNNING MULTICAST MTU:576 Metric:1
# ifconfig vnet0 mtu 1500
# 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
vnet0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
--------------------
So I can at least work around the issue this way after starting the VM,
but still don't understand the root cause.
Joe
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