Monitor Contention Process: Claim Token
Token-Ring operation needs a designated active monitor that performs frame monitoring.
The first adapter that inserts into the ring will always be active monitor.
Station Initialization Process
If a standby monitor detects an active monitor error (no more token frames received; no more AMP frames received) it will initiate the monitor contention process by sending a claim token MAC frame. Claim token MAC frames are sent in intervals of 20 milliseconds.
Theoretically, each ring station may join the contention process in case that the adapter is configured to join it. By default, adapters are configured not to join the process. In any case the very station that sent the initial claim token MAC frame joins the contention process.
Each contending adapter checks passing claim token frames for the source address. If the source address has a higher value than the contending station's MAC address, the claim token frame will pass unchanged and the station stops contending. If contending station's MAC address has a higher value then the frame's source address, the station replaces the frame's source address by its own address. The source station that had sent the claim token frame stops contending because it doesn't receive claim token frames with its own MAC address.
If a contending station receives three claim token frames in sequence containing its own MAC address it will changes from standby monitor to active monitor state. The new active monitor sends a report to the CRS (report new monitor MAC frame). The the ring is purged and a new token is released by the new active monitor.
If the former active monitor stopped operation, its downstream station sends a NAUN change MAC frame (also referred to as SUA change MAC frame).