netdev - Re: [RFC 0/3] VLANs, DSA switches and multiple bridges

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Date:   Thu, 2 Jan 2020 13:54:17 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Pali Rohár <pali.rohar@...il.com>,
        Ido Schimmel <idosch@...sch.org>, netdev@...r.kernel.org,
        Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Vivien Didelot <vivien.didelot@...il.com>
Subject: Re: [RFC 0/3] VLANs, DSA switches and multiple bridges

> # tcpdump -enXXi $host_dsa_interface
> 
> Here's an example ping packet received over the vlan with the above
> configuration, captured from the host DSA interface (ether mac
> addresses obfuscated):
> 
>         0x0000:  DDDD DDDD DDDD SSSS SSSS SSSS dada 0000  .PC.....[h:.....
>                                                ^^^^^^^^^
>         0x0010:  c020 0000 8100 0080 0800 4500 0054 ec40  ..........E..T.@
>                  ^^^^^^^^^ ^^^^^^^^^ ^^^^
>         0x0020:  4000 4001 c314 c0a8 0502 c0a8 0501 0800  @.@.............
>         0x0030:  8784 0c85 0001 32c8 0c5e 0000 0000 59fc  ......2..^....Y.
>         0x0040:  0c00 0000 0000 1011 1213 1415 1617 1819  ................
>         0x0050:  1a1b 1c1d 1e1f 2021 2223 2425 2627 2829  .......!"#$%&'()
>         0x0060:  2a2b 2c2d 2e2f 3031 3233 3435 3637       *+,-./01234567
> 
> dada 0000 c020 0000	- EDSA tag
> 8100 0080		- VLAN ethertype, vlan id
> 0800			- IPv4 ethertype, and what follows is the ipv4
> 			  packet.
> 
> That way it would be possible to know whether the DSA switch is
> forwarding the packets, and in what manner it's forwarding them.

Hi Russell

Try upgrading your tcpdump and pcap library. It now understands DSA
headers, will dump them in a semi human readable way, and also the
rest of the frame.

     Andrew