reliably detecting the presence of a bridge?

James R Cutler james.cutler at
Sat Dec 19 22:53:33 UTC 2015

> On Dec 19, 2015, at 4:53 PM, Larry Sheldon <larrysheldon at> wrote:
> On 12/19/2015 12:17, William Herrin wrote:
> [snip]
>> 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?

Bridge Mode (ATT Passthrough) simply means that the router between the WAN connection and the LAN/WiFi ports is turned off and all ports share the same switch (so packets just “pass through”. Thus all ports appear connected to a common switch.  Call that what you will, there is no spanning tree here even though we all love Radia.

> 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.

The TCP/IP suite was “free” and essentially drove IPX/SPX, DECNet, AppleTalk, and SNA out of any large market.  Digital Equipment Corporation, for example, viewed DECnet networking as a profit center rather than a sales enabler for their hardware and software.  

And nobody wanted to spend the probably very large cost to remove what was essentially network code from applications. This is why almost every application existing need (still needs) modification just to accommodate larger address integers and a different display mechanism for addresses .
> -- 
> sed quis custodiet ipsos custodes? (Juvenal)


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