On Tue, 23 May 2006 10:07:13 +0930
"Mark Zipp" <mark.r.zipp@xxxxxxxxx> wrote:
> Hi,
>
> I'm doing some fault finding where we suspect that the (non-linux)
> device with the fault is generating frames with an MTU / maximum size
> greater than 1500 / 1518 bytes. This device is performing GRE
> tunneling, and it seems that it isn't performing fragmentation or
> PMTUD before it attempts to send a normal, 1500 byte IP packet with
> the GRE header out a GigE interface with a 1500 byte MTU.
>
> I've been asked by the vendor of this device to perform some packet
> captures. As I have a Linux box with a couple of interfaces that
> support > 1500 byte MTUs (an Intel GigE, and a Netgear FA311 (that
> supports an MTU up to 2024 bytes)), I figured I could bridge this
> traffic through this Linux box, and then perform some packet captures
> on the bridge virtual interface.
>
> I set that all up, and it seemed to all be ok. I set both the real
> interface MTUs to 2024 bytes, and when I added them into a bridge, the
> bridge inherited this MTU.
>
> Unfortunately I found that when I attempted to forward frames through
> this bridge configuration, certain sized ones were silently dropped.
> As the devices attached to the bridge shouldn't be generating frames
> with an MTU larger than 1500 (this is our work around solution as a
> reference), it seems that with this 2024 bridge MTU, even normal 1500
> byte MTU frames are dropped.
>
> Through experimentation, by sending packets and changing the MTU of
> the real and bridge interfaces, it seems that the bridge will only
> forward frames up to 1527 bytes MTU. Fortunately I think this will be
> enough to diagnose the fault we are having.
>
> Is this expected behaviour ? A maximum of 1527 bytes seems to be an
> unsual value for the maxiumum supported bridge MTU.
The only bridge related dropping is here in br_forward.c
int br_dev_queue_push_xmit(struct sk_buff *skb)
{
/* drop mtu oversized packets except tso */
if (packet_length(skb) > skb->dev->mtu && !skb_shinfo(skb)->tso_size)
kfree_skb(skb);
Are you doing VLAN's? There was a bug not fixed until 2.6.17 that didn't
account for VLAN header properly.
Are you doing filtering? Some netfilter module might be dropping the packets.
