reliably detecting the presence of a bridge?

Andrew McConachie smutt at
Tue Dec 15 15:04:21 UTC 2015

Flip a bit in the Ethernet FCS as it egresses deviceA. If the frame 
arrives with a correct checksum at deviceB, then there's a switch in the 
middle. Most modern switches recalculate FCS at egress port.

If the frame never arrives, most likely there is a switch in between. If 
the frame arrives with the broken FCS, there is no intermediate switch.


On 12/15/15 4:48 AM, Dave Taht wrote:
> I am curious if there is some sort of igmp or other form of message
> that would reliably detect if a switch had a bridge on it. How could
> deviceA detect deviceC was a bridge in this case?
> deviceA  -> ethernet switch -> deviceB
>                      ethernet switch -> deviceC with bridged wifi and ethernet
> question came up in the context of:
> --
> Dave Täht
> Let's go make home routers and wifi faster! With better software!

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