[186496] in North American Network Operators' Group
Re: reliably detecting the presence of a bridge?
daemon@ATHENA.MIT.EDU (James R Cutler)
Sat Dec 19 17:56:27 2015
X-Original-To: nanog@nanog.org
From: James R Cutler <james.cutler@consultant.com>
In-Reply-To: <5675D1D6.4060903@cox.net>
Date: Sat, 19 Dec 2015 17:53:33 -0500
To: Larry Sheldon <larrysheldon@cox.net>
Cc: nanog@nanog.org
Errors-To: nanog-bounces@nanog.org
> On Dec 19, 2015, at 4:53 PM, Larry Sheldon <larrysheldon@cox.net> =
wrote:
>=20
> On 12/19/2015 12:17, William Herrin wrote:
>=20
> [snip]
>=20
>> 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.
>>=20
>> "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.
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> Bridge specifies a condition that, today, is close enough to always
>> true as makes no difference.
>=20
> Super explanation.
>=20
> But I still have one question (which might be based on errors)--
>=20
> 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.
>=20
> 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?
>=20
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 =E2=80=9Cpass through=E2=80=9D. 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.
>=20
> 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 =E2=80=9Cfree=E2=80=9D 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. =20
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 =
.
=20
> --=20
> sed quis custodiet ipsos custodes? (Juvenal)
Nobody.
