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8.1.2 Ring Administration
The token-passing ring protocol provides relatively greater control and
management at the medium access control (MAC) level than that provided by
the CSMA/CD protocol. All ring administration functions are implemented in the
token-ring adapters and the functions are carried out at the MAC level.
8.1.2.1 Active Monitor
In each operational ring, one station assumes the role of the
active monitor
. The
process of active monitor selection is described in 8.1.2.2, “Token-Claiming
Process” on page 131.
The following are the responsibilities of the active monitor:
•
Maintain the master clock
The active monitor maintains the ring′s master clock. All the other clocks on
the ring are synchronized with the active monitor′s clock.
•
Ensure proper ring delay
To ensure that a token is completely transmitted before returning to the
originating ring station, the active monitor introduces a 24-bit delay (the
length of token) into the ring.
•
Initiating neighbor notification
Every seven seconds, the active monitor broadcasts an Active Monitor
Present (AMP) MAC frame to all the stations on the ring. The first station
that receives this frame, copies it and sets the address-recognized (A) and
frame-copied (C) bit to B′1′. This station, then saves the source address
field from the copied frame as its Nearest Active Upstream Neighbor (NAUN).
After a period of time called
notification-response time
, this station transmits
a Standby Monitor Present (SMP) MAC frame to all the stations on the ring.
The next downstream station ignores the Active Monitor Present MAC frame
(because the A and C bit are set to B′1′), but it copies the Standby Monitor
Present MAC frame issued by its NAUN. It then sets the A and C bits to B′1′
and also copies its NAUN from the source address of this MAC frame. This
station will, in turn, transmit its own Standby Monitor Present MAC frame.
In this way, neighbor notification proceeds around the ring with all the
stations transmitting their own Standby Monitor Present MAC frame until the
active monitor copies the last Standby Monitor Present MAC frame, in which
the A and C bit are set to B′0′. At this point, the neighbor notification is
considered complete.
As can be seen, the neighbor notification (which is also referred to as
ring
poll
) enables each ring station to learn its NAUN address and to provide its
address to its downstream neighbor. This process is used by the TRMM in
the 8250 and the token-ring media modules in the 8260 to perform the
address-to-port mapping
which enables them to determine what is the
address(s) of station(s) attached to each port. The details of address-to-port
mapping are described in 8.10, “Address-to-Port Mapping for Module
Switching Modules” on page 160 and 8.11, “Address-to-Port Mapping for
Per-Port Switching Modules” on page 164.
•
Initiate ring purge process
Occasionally, the active monitor may need to initiate a
ring purge
process to
reset the ring stations, all appropriate timers and release a new token. The
130 8260 Multiprotocol Intelligent Switching Hub