Cisco ASR 9000 Series Service Configuration Guide

Configuring Modular QoS Congestion Management on Cisco ASR 9000 Series Routers
Information About Configuring QoS Congestion Management on Cisco ASR 9000 Series Routers
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Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
OL-23108-02
Modified Deficit Round Robin
MDRR is a class-based composite scheduling mechanism that allows for queueing of up to eight traffic
classes. It operates in the same manner as class-based weighted fair queueing (CBWFQ) and allows
definition of traffic classes based on customer match criteria (such as access lists); however, MDRR does
not use the weighted fair queueing algorithm.
When MDRR is configured in the queueing strategy, nonempty queues are served one after the other.
Each time a queue is served, a fixed amount of data is dequeued. The algorithm then services the next
queue. When a queue is served, MDDR keeps track of the number of bytes of data that were dequeued
in excess of the configured value. In the next pass, when the queue is served again, less data is dequeued
to compensate for the excess data that was served previously. As a result, the average amount of data
dequeued per queue is close to the configured value. In addition, MDRR allows for a strict priority queue
for delay-sensitive traffic.
Each queue within MDRR is defined by two variables:
• Quantum value—Average number of bytes served in each round.
• Deficit counter—Number of bytes a queue has sent in each round. The counter is initialized to the
quantum value.
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.
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.

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English