Re: network bridge default MTU -- apparent change

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



Attachment: signature.asc
Description: OpenPGP digital signature

--
users mailing list
users@xxxxxxxxxxxxxxxxxxxxxxx
To unsubscribe or change subscription options:
https://admin.fedoraproject.org/mailman/listinfo/users
Guidelines: http://fedoraproject.org/wiki/Mailing_list_guidelines

Relevant Pages