Ref: 21770009
Title: Using Spanning Tree Algorithm:  DEC LAN 100 Does Not Work With 3Com
Date: 4/3/91

Copyright 3Com Corporation, 1991.  All rights reserved.

The Spanning Tree Algorithm (STA) is used by bridges on a network to
determine who is the root bridge.

In the IB/2000 and BR/2000, 3Com implements STA according to the IEEE
802.1d specification.  Configuration Bridge Protocol Data Unit (CPBDU)
packets conforming to IEEE 802.1d specifications are multicast by a 3Com
root bridge at every "Hello Time" (configurable in the -STA menu).

If a bridge detects a topology change, and if it is a designated bridge
for the LAN, it sends out a Topology Change Notification BPDU through
its root port.  This information is eventually relayed to the root
bridge which then sets the Topology Change Flag in its CBPDU so that the
information is broadcast to all bridges.  It transmits the CBPDU for a
fixed period of time to ensure that all bridges are informed of the
topology change.

The CPBDU packet is contained in an 802.2 datalink-level packet whose DSAP
and NSAP = 42 with a multicast address of 0180c2000000.  The command
field of the 802.2 frame = 03 (unnumbered information).  The first
octet of the CPBDU packet begins after the 802.2 command field.

Unfortunately, the DEC LAN 100 bridge does not follow 802.1d
specifications.  It uses a proprietary method of communicating root bridge
information and does not work on networks with bridges using 802.1d
specifications for the Spanning Tree Algorithm.

