MVID 05h - Active Monitor Present - AMP
Destination Address: C0-00-FF-FF-FF-FF (ring multicast / 'this ring')
Destination Class: 0 (ring station)
Source Class: 0 (ring station)
Subvectors: SVID 0Bh - Physical Drop Number
In intervals of 7 seconds the active monitor sends AMP MAC frames. This initiates the NAUN process: All ring stations will learn the MAC address of the next addressable upstream neighbor station. After the active monitor had send its AMP frame, all other ring stations will send SMP frames.
AMP = active monitor present
SMP = standby monitor present
NAUN = next addressable upstream neighbor
Each AMP or SMP frame's AC bits are set to zero (A=0, C=0). This will let the next station recognize that it is the following (=next downstream) station relative to the frame's source station (no other station inbetween). The receiving station will copy the frame's source address as NAUN address and set AC=11 (so that following stations won't process the frame, too).
This scheme of sending and receiving AMP/SMP frames is repeated until the active monitor received a SMP frame with AC=11.
If the active monitor doesn't initiate the ring poll process by sending an AMP frame, any other station may start a 'claim token' process.
MVID 03h - Claim Token - CL_TK