[Bridge] MTU/FCS Issue on linux bridge when using SPI interface

[Andy Whiteman]

Hi,

I believe that a bridge set up using bridge-utils is not allowing packets
containing FCS checksums in a specific direction.

I have set up a Raspberry pi with 2 SPI ethernet ports (ENC28J60) and added
them to a bridge. For diagnostics purposes, I also added a USB eth adaptor
to the same bridge.

When frames are going in a very specific direction an FCS is being added
(presumably by the kernel or bridge) however the bridge only seems to allow
1514 bytes to cross it. As soon as FCS is added this goes to 1518 bytes
long and gets dropped by the bridge.
I have increased the MTU settings and decreased them to no avail. I
disabled FCS on the physical chip but also made no change.

Full logs and wire shark screenshots can be found on my issue raised to -
https://github.com/raspberrypi/linux/issues/4488

Ping results from my PC to the pi (host) via different interfaces -
increasing ping data size and monitoring results via wireshark.
PC (10.0.1.20)->SPIEth->Br0->Host (10.0.1.106) No extra bytes - can ping
1472
PC (10.0.1.20)->SPIEth->Br0->Client (10.0.1.135) No extra bytes - can ping
1472
PC (10.0.1.20)->PiGigEth->Br0->Host (10.0.1.106) No extra bytes - can ping
1472
PC (10.0.1.20)->PiGigEth->Br0->USBEth->Client (10.0.1.135) No extra bytes - can
ping 1472
PC (10.0.1.20)->PiGigEth->Br0->SPIEth->Client (10.0.1.135) 4 extra bytes on
response (FCS) - can only ping 1468 (1468-1472 gets dropped 1473 gets
fragmented)
PC (10.0.1.20)->USBEth->Br0->SPIEth->Client (10.0.1.135) 4 extra bytes on
response (FCS) - can only ping 1468 (1468-1472 gets dropped 1473 gets
fragmented)

If I do not have the bridge set up and ping from my PC to the pi (host)
with my PC in the SPI port everything is normal.

Regards
Andy Whiteman
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