From: Choon Lee <[email-masked]>
Date: Wed, 21 Jul 1993 15:27:33 -0700

For the benefit of fellow siglet-ers who has signed on late, I am resending 
this message which was originally sent on July 8 to solicit comments from the 
group. Unfortunately, there has been no response. 

In June's AIW meeting, when a decision was made to form this siglet, there
were 4 people who indicated that they would like to be contributors, and
10 others who would liked to be auditors. The success of this siglet would
depend on all of us.

In the next day or two, I am going to propose some suggestions. In the mean
time, please create some dialogue! Please comment on the following message,
or comment on the scope (charter) of this siglet.

Regards,
Choon Lee
3Com Corp

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To: [email-masked]
Subject: To spanning tree or not to spanning tree
Status: RO

I hope that by now everyone has their mailing set up to begin the discussions
on the topic of loop prevention. To spur discussions, I am throwing out some of 
my thoughts on this topic:

As currently designed, DLSw can be seen as a way of "bridging" LLC2 and 
Netbios traffic between two LANs separated by a IP domain. 
In an environment with a mixture of DLSw and bridges, loops can easily
form. A scheme to resolve the problem of loop formation is needed. 
Whether we wish to leverage off of spanning tree protocol as used in 
transparent bridges is left to be decided.

SPANNING TREE SOLUTION

If we were to use spanning tree protocol across DLSw (by making the DLSw link
an equivalent of a bridging link), and because of the "unequalness" of the links 
(only LLC2 and Netbios get "bridged" across DLSw), we can come across some 
interesting situations, depending on which links get into the "blocking" state.

For example,

      LAN A                         LAN B
	|                             |
	|a ----    b (DLSw)     ---- c|
	|--|  |=================|  |--|
	|  ----                 ----  |
	|                             |
	|  ----                 ----  |
	|--|  |=================|  |--|
	|d ----    e (bridging) ---- f|
	|                             |

if link d, e or f gets into blocking state, all non-LLC2 and non-Netbios
traffic cannot get from one LAN to the other.

Even in the absence of the bridging segment, we can still loose
connectivity between the two LANs in certain configuration,

For example,

      LAN A                         LAN B
	|                             |
	|a ----    b (DLSw)     ---- c|
	|--|  |=================|  |--|
	|  ----                 ----  |
	|                        |    |
	|   ======================    |
	|   |     g (DLSw)            |
	|  ----                 ----  |
	|--|  |=================|  |--|
	|d ----    e (DLSw)     ---- f|
	|                             |

if links c and d goes into blocking state, and since the DLSw nodes do
not do switching between links (i.e. a frame cannot go a-b-g-e-f),
all dlsw-ed traffic between the LANs stops.

Not every network user will use this highly redundant configuration,
yet the threat lingers...

It should be noted that the spanning tree protocol provides the mean to
specify the "cost" of a link. One can theoretically assign a "very high"
cost to the link between the switches, thereby making such links the
preferred ones to go into blocking state. This may solve the above
problems 95% of the time.

NON SPANNING TREE SOLUTION

If we decide not to piggyback on an existing protocol that is basically 
used for preventing loops in bridges (i.e. the spanning tree), we may have
to come up with yet another protocol.

The advantages are:
(1) this protocol needs to be understood only by DLSw nodes - no need
    to tie DLSw to bridging.

(2) since this is a new protocol, it does not have any existing
    pre-condition.

The disadvantages are:
(1) Yet another protocol need to be invented.

(2) Added complexity to DLSw. May need new resources (e.g. functional
    address, sap? ethernet type?)

(3) unproven - no track record.

In the last AIW meeting, IBM indicated that the 6611's implementation 
does not use spanning tree for this purpose. Maybe someone from IBM
can enlighten us on this and provide the group with some of their
thoughts?

/choon lee

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