Why does the *local* token ring network stop functioning when the DSU is disconnected from the MAU? It does not appear that MAUs have any “logic” in them.
It seems from your question that the token ring is unable to go to normal operation when a node is disconnected from the ring. The multi-station access unit may have a physical problem with the particular port; i.e., the port may not be closing its internal circuitry to keep the ring integrity alive.
The other possible scenario is that the ring maintenance procedure may not be working correctly. This could mean the active ring monitor and/or the standby monitor are not reconfiguring the ring upon removal of the DSU. For example, if no token is detected in a fixed time interval of 2.6 seconds, the ring’s standby monitor will enter the monitor contention process, where it wants to be promoted to active monitor. The condition may also be caused by the loss of an Active Monitor Present (AMP) frame on the ring.