Re: Requesting DLT_* values for Ethernet switches proprietary tagging protocol

[Guy Harris <gharris () sonic net>]

On Jan 17, 2019, at 2:39 PM, Florian Fainelli <f.fainelli () gmail com> wrote:

> A number of Ethernet switches from Broadcom, Marvell, Microchip,
> Qualcomm, Lantiq/Intel, etc. utilize proprietary tags that are processed
> by these switches in-line with the Ethernet frame being sent/received.
>
> These tags are inserted by the Ethernet switch's management interface
> for packets egressing the switch to a management port, and are created
> by a software agent (or specialized Ethernet adapter, typically from the
> same vendor) then sent to the management port for frames ingressing the
> switch. These tags serve as the foundation for Linux's Distributed
> Switch Architecture framework. As of Linux 4.19, there is now a way for
> user-space to find out which type of protocol is being
> configured on that management network interface.

So does this mean that Linux is running on a processor in the switch itself, or is this for something where you have a 
Linux box, one of whose Ethernet interfaces is plugged into a management port, with the Linux kernel knowing what type 
of tags are being used on that port, and with 4.19 exporting that information to userland?

And does the comment in your tcpdump change


        /* p points to after the Ethernet header (include type/length), move it
         * backwards by 2 bytes to parse the Broadcom tag and extract the actual
         * type/length after it.
         */

mean that the Broadcom tags, at least, are inserted between the source MAC address and type/length field (so that the 
frames have a modified header containing the tag, rather than having a trailer containing the tag, for example)?
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