Subject: no can do

From: Choon Lee <[email-masked]>
Date: Thu, 12 Aug 1993 15:59:16 -0700

it hit me suddenly that there are times that a "loop" should
not be broken. An example is the triangular topology (full mesh)
case:

	  LAN A
	   ========
	       |
            -------
            |     |
            |     |
            -------
             |   |
             |   |
  -------    |   |     -------
  |     |-----   ------|     |
  |     |--------------|     |
  -------              -------
     |                    |
  =======              ========
  LAN B                   LAN C

Spanning tree would detect a "loop" in the triangular topology, and
will attempt to block one of the links. Since DLSw does not cascade,
the network is broken!

A case of spanning tree working too well! :-)

Regards,
Choon Lee
3Com Corp

---

From: [email-masked] (Scott Adams)
Date: Fri, 13 Aug 1993 10:40:22 -0400 (EDT)

Hi Choon Lee,

this is Scott Adams of Cayman Systems.

You've addressed here an issue that I've found a little confusing for
some time...that is, what exactly are we trying to prevent loops in?

> not be broken. An example is the triangular topology (full mesh)

I've been assuming that whatever loop prevention method we use
wouldn't block data traffic on any TCP connections, but insteaad it
would just prevent broadcast UI frames (as SSP CANUREACH, NETBIOS_NQ
and NETBIOS_ANQ) from being circulated endlessly around a DLSw
network.  Since DLSw are supposed to be connected in a meshed manner
(based on my understanding that DLSw's don't cascade, as you write
above, which I interpret as they don't switch traffic between DLSw
Tunnels - my term for the connections between DLSw's), it wouldn't
make sense to block Data Links across any DLSw Tunnel.

In fact, having thought about this more, if all we're trying to do is
to prevent broadcast frames from endlessly circulating through the
DLSw mesh, then there is an easy solution.  If DLSw's never
re-transmit over a DLSw Tunnel CANUREACH, NETBIOS_NQ AND NETBIOS_ANQ
packets received on another DLSw Tunnel, then there would be no
looping problem.  I can't believe that there can be so simple a
solution, so I'm probably just forgetting something fundamental here.
I'm going to stop and go over various connection scenarios and see if
it works...or figure out what I'm missing.

> 3Com Corp

ditto,

Scott Adams