Token Ring Bridge/Ring Numbering and Routing Avoids Looping (102373)

SUMMARY

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).