Hi all

I'm wondering what is the behavior on a bridge with different MTU sizes on its devices. For example, let's assume the following configuration :
eth1 : mtu = 1200
eth2 : mtu = 1500

if I set br0 as a bridge of eth1 (inbound) and eth2 (outbound), then br0 will take the lower mtu :
bd0: mtu=1200
eth1 : mtu = 1200
eth2 : mtu = 1500

So I'm wondering why eth2's mtu isn't automatically reduced to 1200 by the system ? and above all, if this 1500 mtu is used somewhere during the packets communications or if the system will force all the packets incoming and outgoing on that device to mtu 1200 ?

Thank you in advance
Best regards
julien

Reducing the MTU of eth2 locally does not affect the size of incoming frames (the network architecture still supports an MTU of 1500 bytes), so you may as well leave it unchanged.

Packets larger than 1200 bytes will be dropped by the bridge, and it's up to the upper layers of the stack of the transmitting host to detect this and adjust the path MTU accordingly.

1 members found this post helpful.

Thank you for your answer.
In addition, I've found that all devices share the same mtu size. The bridge doesn't fragment packets.
Also, it seems that this code has only been implemented since kernel 2.6 :
http://lists.linuxfoundation.org/pip...ly/004054.html

Thanks
Julien