Cisco CATALYST 3920 24 PORT TOKEN RING SWITCH Specification Guide
Product Overview
Catalyst 3920 Token Ring Switch User Guide
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In addition, two switches may be interconnected by a number of parallel Token Ring connections,
called TokenChannels. A TokenChannel is two to eight parallel links between Catalyst 3920
switches. The traffic between the switches is shared via the connections.
called TokenChannels. A TokenChannel is two to eight parallel links between Catalyst 3920
switches. The traffic between the switches is shared via the connections.
For more information about stacking Catalyst switches and using TokenChannels, refer to the
“Interconnecting Catalyst 3920 Switches” chapter.
“Interconnecting Catalyst 3920 Switches” chapter.
Bridging Modes
The Catalyst 3920 supports the following bridging and switching modes:
•
Source-Route Bridging
•
Source-Route Transparent Bridging
•
Source-Route Switching
For more information about these bridging modes, refer to the “Understanding Token RingSwitching” appendix.
Token Ring Port Operation
Each of the 24 Token Ring ports can operate in one of the following modes:
•
Half-duplex concentrator port—The port is connected to a single station in HDX mode. In this
case, the port behaves like an active media access unit (MAU) port for classical Token Ring.
case, the port behaves like an active media access unit (MAU) port for classical Token Ring.
•
Half-duplex station emulation—The port is connected to a port on an MAU. In this case, the port
behaves like a station connected to a classical Token Ring segment that contains multiple
stations.
behaves like a station connected to a classical Token Ring segment that contains multiple
stations.
•
Full-duplex concentrator port—The port is connected to a single station in FDX mode.
•
Full-duplex station emulation—The port is connected to another Token Ring switch in FDX
mode.
mode.
The mode of operation can be configured, or it can be automatically sensed when equipment is
connected to the port. The media speed (4 or 16 Mbps) can also be configured or automatically
sensed in all port modes.
connected to the port. The media speed (4 or 16 Mbps) can also be configured or automatically
sensed in all port modes.
Transmission Priority Queues
To address the needs of delay-sensitive data, such as multimedia, the Token Ring ports of the
Catalyst 3920 have two transmit queues, a high-priority queue and a low-priority queue.
Catalyst 3920 have two transmit queues, a high-priority queue and a low-priority queue.
The queue for a frame is determined by the value of the priority field in the frame control (FC) byte
of the frame. If FC priority is above a configurable level (the default is 3), the frame is put in the
high-priority queue. If an output port becomes congested, you can dynamically configure the port to
transmit all frames at high priority regardless of the FC byte contents.
of the frame. If FC priority is above a configurable level (the default is 3), the frame is put in the
high-priority queue. If an output port becomes congested, you can dynamically configure the port to
transmit all frames at high priority regardless of the FC byte contents.
The Catalyst 3920 CPU software monitors the size of the output queue at each Token Ring port to
minimize the effects of congestion at output ports. When port congestion is detected, the
Catalyst 3920 does the following:
minimize the effects of congestion at output ports. When port congestion is detected, the
Catalyst 3920 does the following:
•
Raises the transmit priority to a higher level for low-priority frames
•
Discards the oldest frames when the output queue is almost full