Brocade Communications Systems 53-1002163-02 Ethernet pause, Changing the Tail Drop threshold

11 Congestion control

consume the full buffer memory. Thresholds can also be used to bound the maximum queueing delay for each traffic class. Additionally if the sum of the thresholds for a port is set below 100 percent of the buffer memory, then you can also ensure that a single port does not monopolize the entire shared memory pool.

FIGURE 7 Queue depth

The tail drop algorithm can be extended to support per priority drop thresholds. When the ingress port CoS queue depth breaches a threshold, then any frame arriving with the associated priority value will be dropped. Figure 7 describes how you can utilize this feature to ensure lower priority traffic cannot totally consume the full buffer memory. Thresholds can also be used to bound the maximum queueing delay for each traffic class. Additionally if the sum of the thresholds for a port is set below 100 percent of the buffer memory then you can also ensure that a single CoS does not monopolize the entire shared memory pool allocated to the port.

Changing the Tail Drop threshold

To change the Tail Drop threshold, perform the following steps from privileged EXEC mode. 1. Enter global configuration mode.

switch#configure terminal

2.Change the Tail Drop threshold for each multicast traffic class. In this example, 1000pkt is used.

switch(config)#qos rcv-queue multicast threshold 1000 1000 1000 1000

3. Enter the copy command to save the running-config file to the startup-config file.

switch(config)#do copy running-config startup-config

Ethernet pause

Ethernet Pause is an IEEE 802.3 standard mechanism for back pressuring a neighboring device. Pause messages are sent by utilizing the optional MAC control sublayer. A Pause frame contains a 2-byte pause number, which states the length of the pause in units of 512 bit times. When a device receives a Pause frame, it must stop sending any data on the interface for the specified length of

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