[Bridge] Small packets

Stephen Hemminger
Sun, 28 Oct 2007 22:12:32 -0000

On Mon, 29 Oct 2007 15:37:52 +1100
"Leigh Sharpe" <lsharpe@pacificwireless.com.au> wrote:

> Hi All,
>  I have a bridge set up thusly:
>
>
> brctl addbr br0                                 # Create the bridge.
>
> brctl addif br0 eth2                            # Add eth2
> brctl addif br0 eth3                            # And eth3
>
> brctl setfd br0 0                               # Forward immediately.
>
> brctl stp br0 off                               # Turn off Spanning
> Tree.
>
> ifconfig eth2 up                                # bring up eth2
> ifconfig eth3 up                                # Bring up eth3
> ifconfig br0 up                                 # Bring up the bridge.
>
> Which seems to bridge OK. However, I have noticed that the bridge
> appears to be truncating some packets. In particular, I am worried
> about packets such as IP ACK's, which are entering the bridge as
> 64-byte packets, but coming out the other side as 60-byte packets.
> As I recall, the smallest valid ehternet packet is 64 bytes. The
> bridge seems to be removing any padding in the packet and sending it
> on it's way as a packet smaller than the minimum ethernet packet
> size. Looking at the packet distribution on other devices connected
> to the bridge, it seems to be doing the same to lots of other
> packets, too. Anybody got any idea why the bridge might be truncating
> packets? Any idea why it's sending 60-byte packets, when the minimu
> should be 64? Or where to tell it to pad all packets out to 64 bytes?
>
>
> Leigh.
>
>
>
>
> Regards,
>              Leigh
>
> Leigh Sharpe
> Network Systems Engineer
> Pacific Wireless
> Ph +61 3 9584 8966
> Mob 0408 009 502
> Helpdesk 1300 300 616
> email lsharpe@pacificwireless.com.au
> <blocked::mailto:lsharpe@pacificwireless.com.au>
> web www.pacificwireless.com.au
> <blocked::http://www.pacificwireless.com.au/>
>

What hardware? It could be a driver bug.
Also if you use VLAN's then the driver may send 60 byte packet (because
of the 4 byte VLAN tag).