Subject: question on spanning tree and DLSw

From: [email-masked]
Date: Fri, 30 Jul 93 08:21:40 -0400

I am having some difficulty understanding how a DLSw node could participate in
the spanning tree algorithm without denying service to some users that are only
reachable through the DLSw network.

In determining whether or not spanning tree can be used, we should consider
how it affects SDLC connectivity in the following environment:

    2 Token Rings (R1 and R2) are connected by remote bridging and also by a
    pair of DLSw nodes (DLSw1 is on R1 and DLSw2 is on R2); the DLSw1 node
    has some SDLC devices that normally talk to Token Ring devices on R2

In this case, would the link between DLSw1 and DLSw2 normally be blocked
because the remote bridging would be the preferred path?  If so, this
would deny service to the SDLC devices on the DLSw1 node.

How would a case where the DLSw2 node provides the only connection to another 
token ring (R3) that devices on R1 and the DLSw1 node need to talk to be
handled?

Norm Rehn

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From: [email-masked] (John R. Pickens)
Date: Fri, 30 Jul 93 17:52:51 PDT

Your questions remind me to make the following comment.  If this has
already been covered by previous dialogue, then forgive the
redundancy.

The purpose of spanning tree is to prevent loops.  A side effect of
having spanning tree is that only a subset of paths can be traversed.
But, as a result, loops do not occur.  If you want the ability to use
multiple paths, then you need to use routing.  An APPN router node
with SDLC attached will suffer none of the problems you highlight.

Also, it is important that the loop prevention mechanism used 
by DLSW also participate in, or be aware of, the bridging spanning
tree.  It makes no sense to enable a loop-free (single) DLSW path, if a 
parallel bridged path also exists.

Also, parallel bridged paths can also occur in the local environment
(perhaps the more common case).  Just consider two hubs connected
via bridges (or perhaps bridging per port).  It is easy to create
loops in the local environment.

(Choon probably already said all of the above previously.  :-} )

JP

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