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Congestion Management. Configuring LAN Congestion Management. Multiple queues protect the queue containing important traffic (voice) from drops. The number of queues available depends upon the switch model and port type. On some switches, “drop thresholds” can be assigned to each queue.
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Congestion Management Configuring LAN Congestion Management
Multiple queues protect the queue containing important traffic (voice) from drops. The number of queues available depends upon the switch model and port type. On some switches, “drop thresholds” can be assigned to each queue. On some switches, queues can have normal tail drop or WRED dropping. Drops happen in data-only queue(s). Queuing on Catalyst Switches
Queuing on Catalyst Switches (Cont.) • Key queuing features depend upon the switch hardware: • The number of queues per port • The type of queues (priority or standard) • The capability to have drop thresholds for a queue • The number of drop thresholds per queue • The type of drop thresholds (tail drop or WRED) • Switch queuing capabilities are shown as: • 2Q2T: • Two queues • Two drop thresholds for each queue • 1P2Q2T: • One priority queue • Two additional queues • Two drop thresholds for each queue
Queuing on Catalyst Switches (Cont.) Catalyst 2950 Switches • 4 transmit queues (1P3Q or 4Q) • Need to configure PQ and ensure that CoS 5 traffic is assigned to the PQ • Configurable PQ for queue 4 • Configurable CoS to specific queue • Configurable queue weight
Weighted Round Robin • WRR overcomes the problem of having PQ starving out the lower priority queues. • WRR scheduling prevents queues with a lower weight from being completely starved during periods of heavy high-priority traffic. • Different weights are assigned to each queue. • For example, in one scheduling round, the WRR scheduler will transmit: • Three frames from a queue assigned weight 3 • Four frames from a queue assigned weight 4 • WRR with an expedite queue: When WRR is configured on a Catalyst 2950, the option exists to configure queue 4 as a priority queue—an “expedite queue.”
Configuring PQ on Catalyst 2950 Switches Switch(config)# wrr-queue cos-map quidcos1...cosn • Assigns CoS values to CoS priority queues • quid: Specifies the queue ID of the CoS priority queue. (Ranges are 1 to 4 where 1 is the lowest CoS priority queue.) • cos1...cosn: Specifies the CoS values that are mapped to the queue ID. • Default ID values are: • Queue IDCoS Values • 1 0, 1 • 2 2, 3 • 3 4, 5 • 4 6, 7
Configuring WRR on Catalyst 2950 Switches Switch(config)# wrr-queue bandwidth weight1...weight4 • Assigns WRR weights to the four egress queues • Ranges for the WRR values: • For weight1,weight2, and weight3, the range is 1 to 255. • For weight4, the range is 0 to 255 (when weight4 is set to 0, queue 4 is configured as the expedite queue). ! Queueing Configuration is done globally on the Catalyst 2950 ! wrr-queue bandwidth 20 1 80 0 no wrr-queue cos-map wrr-queue cos-map 1 0 1 2 4 wrr-queue cos-map 3 3 6 7 wrr-queue cos-map 4 5 ! interface GigabitEthernet0/12
Monitoring Queuing on Catalyst 2950 Switches Switch> show mls qos maps [cos-dscp | dscp-cos] • Displays QoS mapping information. • This command is available with enhanced software image switches. Switch> show mls qos maps Dscp-cos map: dscp: 0 8 10 16 18 24 26 32 34 40 46 48 56 ----------------------------------------------- cos: 0 1 1 2 2 3 3 4 4 5 5 6 7 Cos-dscp map: cos: 0 1 2 3 4 5 6 7 -------------------------------- dscp: 0 8 16 24 32 40 48 56
Monitoring Queuing on Catalyst 2950 Switches (Cont.) Switch> show wrr-queue bandwidth • Displays the WRR bandwidth allocation for the CoS priority queues Switch> show wrr-queue bandwidth WRR Queue : 1 2 3 4 Bandwidth : 10 20 30 40 Switch> show wrr-queue cos-map • Displays the mapping of the CoS priority queues Switch> show wrr-queue cos-map CoS Value : 0 1 2 3 4 5 6 7 Priority Queue : 1 1 2 2 3 3 4 4
Monitoring Queuing on Catalyst 2950 Switches (Cont.) Switch> show mls qos interface [interface-id] [policers] • Displays QoS information at the interface level Switch> show mls qos interface fastethernet0/1 FastEthernet0/1 trust state:trust cos trust mode:trust cos COS override:dis default COS:0 pass-through:none trust device:cisco-phone
Summary • The number and capabilities of queues on Catalyst switches depend upon the model of the switch, supervisor, and line cards. PQ and WRR are the two queuing methods used for Catalyst switches. The use of PQ can starve lower-priority queues. • With WRR, different weights are assigned to each queue. Use of WRR scheduling prevents the low-priority queues from being completely neglected during periods of high-priority traffic. On most Catalyst switches, a single priority queue can be configured with WRR to ensure priority dispatch of voice traffic. • To configure CoS-to-queue mappings for PQ on the Catalyst 2950 switch, specify the queue ID of the CoS priority queue. (Ranges are 1 to 4 where 1 is the lowest CoS priority queue.) Then, specify the CoS values that are mapped to the queue ID. Use the wrr-queue cos-map quid cos1...cosn command.
Summary (Cont.) • The wrr-queue bandwidth global configuration command is used to assign WRR weights to the four CoS priority queues on the Catalyst 2950 switch. • The show mls qos maps command is used to display QoS mapping information on the Catalyst 2950 switch.
