DOCUMENT:Q102373  30-JUL-2001  [lanman]
TITLE   :Token Ring Bridge/Ring Numbering and Routing Avoids Looping
PRODUCT :Microsoft LAN Manager
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OPER/SYS:
KEYWORDS:

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SUMMARY
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On a token ring topology, all bridges and rings are identified by a unique
number (4 bits for bridges, 12 bits for rings). Before forwarding a frame, a
bridge adds its bridge number and the ring number of the next ring to the source
routing information built up in the media access control header. The first
bridge to forward also adds the originating ring's number. Other bridges decide
whether to forward a frame by checking if the ring on the other side is already
in the source routing information for that field--if it is, they do not forward
it.

Forwarding is complicated somewhat by a distinction between single- route and
general (aka all-route) broadcasts. A single-route broadcast is forwarded only
by bridges that are defined to be in the spanning tree for rings, which can be
defined manually or automatically by a bridge-to-bridge protocol. General route
broadcasts are forwarded by all bridges, subject to the restriction defined in
the first paragraph.

So, if you had three bridges A, B, and C in a ring, two of them would be defined
as in the spanning tree and the other one wouldn't, and a single route broadcast
wouldn't loop. A general broadcast would go around in both directions (therefore
appearing on rings B and C twice) but would not be forwarded back onto A.

All NetBIOS broadcast frames (datagrams, name query, remote adapter status) are
sent as single-route broadcasts, so they will appear on a given ring only once
(unless the spanning tree is misconfigured).

REFERENCES
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IBM Token-Ring Network Architecture Reference, IBM part # SC30- 3374-02 39F9354.

Additional query words: 2.00 2.0 2.10 2.1 2.10a 2.1a 2.20 2.2

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