Data Communications Networking Devices - 4th Ed.pdf

344 ________________________________________________________ LOCAL AREA NETWORKSframes are circulating on the ring but the active monitor is operating, the standbymonitor might falsely presume the active monitor has failed. Thus, the activemonitor will periodically issue an active monitor present AMP) MAC frame. Thisframe must be issued every seven seconds to inform the standby monitors that theactive monitor is operational. Similarly, standby monitors periodically issue astandby monitor present SMP) MAC frame to denote they are operational.If an active monitor fails to send an AMP frame within the required timeinterval, the standby monitor with the highest network address will continuouslytransmit claim token CL_TK) MAC frames in an attempt to become the activemonitor. The standby monitor will continue to transmit CL_TK MAC framesuntil one of three conditions occurs: A MAC CL_TK frame is received and the sender's address exceeds the standbymonitor's station address. A MAC beacon BCN) frame is received. A MAC purge PRG) frame is received.If one of these conditions occurs, the standby monitor will cease its transmission ofCL_TK frames and resume its standby function.Purge frameIf a CL_TK frame issued by a standby monitor is received back withoutmodi®cation and neither a beacon nor a purge frame is received in response to theCL_TK frame, the standby monitor becomes the active monitor and transmits apurge MAC frame. The purge frame is also transmitted by the active monitor eachtime a ring is initialized or if a token is lost. Once a purge frame is transmitted, thetransmitting device will place a token back on the ring.Beacon frameIn the event of a major ring failure, such as a cable break or the continuoustransmission by one station known as jabbering), a beacon frame will be transmitted.The transmission of BCN frames can be used to isolate ring faults. For anexample of the use of a beacon frame, consider Figure 3.40 in which a cable faultresults in a ring break. When a station detects a serious problem with the ring, suchas the failure to receive a frame or token, it transmits a beacon frame. That framede®nes a failure domain which consists of the station reporting the failure via thetransmission of a beacon and its nearest active upstream neighbor NAUN), as wellas everything between the two.If a beacon frame makes its way back to the issuing station, that station willremove itself from the ring and perform a series of diagnostic tests to determine if itshould attempt to reinsert itself into the ring. This procedure ensures that a ringerror caused by a beaconing station can be compensated for by having that stationremove itself from the ring. Since beacon frames indicate a general area where afailure occurred, they also initiate a process known as auto-recon®guration. The