[186389] in North American Network Operators' Group
Re: reliably detecting the presence of a bridge?
daemon@ATHENA.MIT.EDU (Andrew McConachie)
Tue Dec 15 20:00:46 2015
X-Original-To: nanog@nanog.org
To: Dave Taht <dave.taht@gmail.com>
From: Andrew McConachie <smutt@depht.com>
Date: Tue, 15 Dec 2015 10:04:21 -0500
In-Reply-To: <CAA93jw6LXBpSfXcF2PgqrLjQ53bOdwXErHZNpjMpmEJ-N7FKVw@mail.gmail.com>
Cc: NANOG <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
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.
--Andrew
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:
>
> http://lists.alioth.debian.org/pipermail/babel-users/2015-December/002231.html
>
> --
> Dave Täht
> Let's go make home routers and wifi faster! With better software!
> https://www.gofundme.com/savewifi
