reliably detecting the presence of a bridge?

Larry Sheldon larrysheldon at
Sat Dec 19 21:53:26 UTC 2015

On 12/19/2015 12:17, William Herrin wrote:


> I recommend you stop using the word "bridge." I think see where you're
> heading with it, but I think you're chasing a blind alley which
> encourages a false mental model of how layer 2 networks function. You
> came here for answers. This is one of them.
> "Bridge" describes a device which existed in layer 2 networks a
> quarter century ago. You had a 10-base2 ethernet with every station
> connected to a shared coax wire. Or you had a token ring where each
> station was wired to the next station in a loop. Or if you were
> sophisticated you had 10-baseT with a hub that repeated bits from any
> port to all ports with no concept of packets.
> And then you had a bridge which could connect these networks together,
> buffering complete packets and smartly repeating only the packets
> which belong on the other side. The bridge let you expand past the
> distance limitations imposed by the ethernet collision domain, and it
> let you move between two different speed networks.
> These networks are now largely a historical curiousity. There are no
> hubs, no 10-base2, no token passing rings. Not any more. Individual
> stations now connect directly to a bridge device, which these days we
> often call a "switch." Even where the stations have a shared media
> (e.g. 802.11), the stations talk to the bridge, not to each other.
> Bridge specifies a condition that, today, is close enough to always
> true as makes no difference.

Super explanation.

But I still have one question (which might be based on errors)--

I think I have used WiFi terminals ("air ports", "WiFi routers" [spit]) 
that offer a "bridge" mode, apparently to build a dedicated radio link 
between two such terminals.

Are they operating as a Radia Perlman "bridge", or is this yet another 
example if the Wiffy World high-jacking words and terms that used to 
have actual meanings?

Nice write-up, even though it is sort of sad to be confronted with the 
fact that my experience and knowledge with hose-connected (10base5. 
10base2) or token-ring networks, and hubs, and stuff is now without 
value.  That is the very worst part of getting old.

Next objective:  Somebody to 'splain at what happened to the 
wonderfulness of the OSI model where layer X did not know, could not 
know, did not care what layer X-1 was, did, or how it did it.
sed quis custodiet ipsos custodes? (Juvenal)

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