Brocade Converged Enhanced Ethernet Administrator's Guide v6.1.2_cee (53-1001258-01, June 2009)

Converged Enhanced Ethernet Administrator’s Guide 111
53-1001258-01
Congestion control
8
The basic tail drop algorithm does not have any knowledge of multiple priorities and per traffic
class drop thresholds can be associated with a queue to address this. When the queue depth
breaches a threshold, then any packet arriving with the associated priority value will be dropped.
Figure 8 shows how you can utilize this feature to ensure that 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% of
the buffer memory, then you can also ensure that a single port does not monopolize the entire
shared memory pool.
FIGURE 8 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 packet arriving with the associated priority
value will be dropped. Figure 8 shows 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% 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
Example of increasing multicast packet expansion Tail Drop Threshold to 1000pkt for each multicast Traffic Class.
switch:admin>cmsh
switch>enable
switch#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)#qos rcv-queue multicast threshold 1000 1000 1000 1000
switch(config)#exit
Step Task Command
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