Comment 2 for bug 1403955

https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1355813/comments/3 might be relevant here. Kapil says: "...you can't really set a bridge mtu, you have to set it for all the containers on the bridge, ie. bridges take the lowest mtu of their interfaces. so instead we have to set all the containers mtu."

If so, I'm not sure it really makes sense for the DHCP client to set the MTU on eth0 when receiving DHCP over br0. I suppose it depends on the use case though.