Low-Latency Queueing With Strict Priority Queueing; Traffic Shaping - Cisco ASR 9000 Series Configuration Manual

Information About Configuring QoS Congestion Management on Cisco ASR 9000 Series Routers
Packets in a queue are served as long as the deficit counter is greater than zero. Each packet served
decreases the deficit counter by a value equal to its length in bytes. A queue can no longer be served after
the deficit counter becomes zero or negative. In each new round, the deficit counter for each nonempty
queue is incremented by its quantum value.
In general, the quantum size for a queue should not be smaller than the maximum transmission unit
Note
(MTU) of the interface to ensure that the scheduler always serves at least one packet from each nonempty
queue.

Low-Latency Queueing with Strict Priority Queueing

The LLQ feature brings strict priority queueing (PQ) to the MDRR scheduling mechanism. PQ in strict
priority mode ensures that one type of traffic is sent, possibly at the expense of all others. For PQ, a
low-priority queue can be detrimentally affected, and, in the worst case, never allowed to send its packets
if a limited amount of bandwidth is available or the transmission rate of critical traffic is high.
Strict PQ allows delay-sensitive data, such as voice, to be dequeued and sent before packets in other
queues are dequeued.
LLQ enables the use of a single, strict priority queue within MDRR at the class level, allowing you to
direct traffic belonging to a class. To rank class traffic to the strict priority queue, you specify the named
class within a policy map and then configure the priority command for the class. (Classes to which the
priority command is applied are considered priority classes.) Within a policy map, you can give one or
more classes priority status. When multiple classes within a single policy map are configured as priority
classes, all traffic from these classes is enqueued to the same, single, strict priority queue.
Through use of the priority command, you can assign a strict PQ to any of the valid match criteria used
to specify traffic. These methods of specifying traffic for a class include matching on access lists,
protocols, IP precedence, and IP differentiated service code point (DSCP) values. Moreover, within an
access list you can specify that traffic matches are allowed based on the DSCP value that is set using the
first six bits of the IP type of service (ToS) byte in the IP header.

Traffic Shaping

Traffic shaping allows you to control the traffic flow exiting an interface to match its transmission to the
speed of the remote target interface and ensure that the traffic conforms to policies contracted for it.
Traffic adhering to a particular profile can be shaped to meet downstream requirements, thereby
eliminating bottlenecks in topologies with data-rate mismatches.
To match the rate of transmission of data from the source to the target interface, you can limit the transfer
of data to one of the following:
•
•
The rate of transfer depends on these three components that constitute the token bucket: burst size, mean
rate, and time (measurement) interval. The mean rate is equal to the burst size divided by the interval.
When traffic shaping is enabled, the bit rate of the interface does not exceed the mean rate over any
integral multiple of the interval. In other words, during every interval, a maximum of burst size can be
sent. Within the interval, however, the bit rate may be faster than the mean rate at any given time.
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
QC-40
Configuring Modular Quality of Service Congestion Management on Cisco ASR 9000 Series Routers
D r a f t — C i s c o C o n f i d e n t i a l
A specific configured rate
A derived rate based on the level of congestion
OL-17239-01