Configuring Tail Drop; Restrictions - Cisco ASR 9000 Series Configuration Manual

Configuring Modular QoS Congestion Avoidance on Cisco ASR 9000 Series Routers

Configuring Tail Drop

Packets satisfying the match criteria for a class accumulate in the queue reserved for the class until they
are serviced. The queue-limit command is used to define the maximum threshold for a class. When the
maximum threshold is reached, enqueued packets to the class queue result in tail drop (packet drop).
The queue-limit value uses the guaranteed service rate (GSR) of the queue as the reference value for the
queue_bandwidth. If the class has bandwidth percent associated with it, the queue-limit is set to a
proportion of the bandwidth reserved for that class.
If the GSR for a queue is zero, use the following to compute the default queue-limit:
•
•
The default queue-limit is set to bytes of 100 ms of queue bandwidth. The following formula is used to
Note
calculate the default queue limit (in bytes):
bytes = (100 ms * queue_bandwidth kbps) / 8

Restrictions

When configuring the queue-limit command in a class, you must configure one of the following
commands: priority, shape average, bandwidth, or bandwidth remaining, except for the default class.
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OL-17239-01
D r a f t — C i s c o C o n f i d e n t i a l
1 percent of the interface bandwidth for queues in a nonhierarchical policy.
1 percent of minimum parent shape and interface rate for queues within a hierarchical policy.
configure
policy-map policy-name
class class-name
queue-limit value [units]
priority
class class-name
bandwidth {bandwidth [units] | percent value}
exit
exit
interface type instance
service-policy {input | output} policy-map
end
or
commit
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
How to Configure Modular QoS Congestion Avoidance on Cisco IOS XR Software
QC-73