Understanding the network.pdf - Back to Home

management, but also ring station configuration and monitoring services, ring error
data collection, and centralized network management. Therefore, in addition to the
active and standby monitor functions, the IBM Token Ring architecture defines
additional management servers to provide the needed additional functionality.
These additional Token Ring management servers work primarily in conjunction
with IBM's LAN Network Manager product.
The primary ring station management role is known as the active monitor (AM). The
AM is responsible for maintaining the balanced flow of management and endstation
data over the ring. Each ring has one station that serves as the AM. Any
station on the ring can assigned the role of the AM, which is determined through the
token claiming process. The token claiming process is initiated when the first station
joins the ring and whenever it is perceived that no active monitor exists on the ring,
by any of the attached ring stations. The token claiming process (for example, the
AM "election"), is won by the station with the highest active MAC address on the ring.
The following list details the AM's ring management functions:
• Neighbor notification—The AM is responsible for initiating and monitoring the
neighbor notification process. The ring's data flow rotation travels only in
one direction. In order for the ring to manage and identify error conditions,
each ring station needs to know the MAC address for its nearest active
upstream neighbor (NAUN). Because data on the ring flows in only one
direction, the downstream neighbor detects any errors transmitted by the
upstream neighbor. In the event of an error condition, the section of the ring
between the NAUN and the station detecting the error is considered the fault
domain (in most cases). Depending on the error, the stations within the fault
domain can enter an error condition known as beaconing, which alerts the
other stations on the ring that a ring fault error has occurred. Stations on the
ring learn the address of their NAUN through the neighbor notification
process. The AM transmits an active monitor present (AMP) frame to each
station at least once every 15 seconds. As each station receives the AMP, it
responds with a standby monitor present (SMP) message to notify its
downstream neighbor of its presence and MAC address.
• Ring recovery—During some types of ring error conditions, the ring needs to
be "reset" to its normal operational state. This is accomplished through the
ring purge procedure. To purge the ring, AM transmits a ring purge token
onto the ring. As each station receives the token, it resets itself and initiates
the neighbor notification process.
• Token monitoring—The Token Ring specification dictates that it should take
no more then 10msec for a token to traverse the ring. If the AM fails to see
a good token within this period, the AM initiates ring recovery. Additionally,
the AM interrogates each token that is transmitted on the ring. The AM does
this by checking the frame status bit. If the frame status bit is incorrect, the
AM interprets the token as errored and initiates a ring purge.