CANOGA PERKINS 9175 Configuration Guide - Manual (Page 224)

CanogaOS Configuration Guide
32-1
32 Configuring Hierarchical QoS
32.1 Overview
The switch supports a hierarchical QoS configuration that is applied to the service port
and network port. Hierarchical QoS configuration is based on the concept of a
bandwidth-limited stream of traffic, which is a stream of packets that has its departure
rate constrained in some manner. The features, including traffic classification, queuing,
shaping, and scheduling, are involved in the hierarchical QoS configurations. You can
create a hierarchy by simply applying a QoS SAP egress policy to an EVC in the egress
UNI or a network egress policy to an EVC in the egress NNI.
In the QoS SAP egress and network egress policies, you configure the bandwidth for
high priority queues, the total limited rate for a specific EVC, the proportion of sharing
residual bandwidth for best-effort queue, the WTD threshold for each service queue.
When an EVC is successfully associated with a QoS policy, the switch allocates
exclusive service queues for the EVC in both egress UNI and egress NNI directions.
32.2 Terminology
Following is a brief description of terms and concepts used to describe hierarchical QoS:
Exclusive Queues for EVC
The switch allocates two sets of service queues for each EVC, each set with four queues,
used for egress UNI and egress NNI respectively. In each set, service queues are
treated with different preference. Queue 0 is also alias as best-effort queue for
backlogging service stream with lowest priority. Queue 3 owns the highest priority and
always obtains the best preferential treatment when dealing with scheduling. Following
this rule, Queue 2 and Queue 1 acquires sub-preferential treatment. The service queue
which a service packet destines to is prescribed by the priority assigned to the packet.
Service packet with priority 0 through 15 will destine to queue 0, with priority 16 through
31 will destine to queue 1, with priority 32 through 47 will destine to queue 2, and with
priority 48 through 63 will destine to queue 3. The priority assigned to a packet can be
influenced by using the qos map cos-pri-color global configuration command and the set
priority policy-map configuration command. For information about the QoS priority,
please refer to “Configuring QoS” chapter.
Bandwidth Limitation for EVC
The bandwidth limitation for EVC can be easily realized by associating with an SAP
egress policy and a network egress policy. The SAP egress policy defines a set of QoS
rules for rate-limiting the queue bandwidth and the total bandwidth of an EVC at the
egress UNI. Similarly, the network egress policy defines a set of QoS rules for
rate-limiting the queue bandwidth and the total bandwidth of an EVC at the egress NNI.
Additionally, in these egress policies, WTD (Weighted Tail Drop) threshold is configured
for each queue. Packets will be backlogged in the queues if the instantaneous incoming
rate exceeds the configuraed allowable bandwidth and will be get dropped until the
queue length exceeds the WTD threshold. For information about WTD threshold, please
refer to the “Congestion Avoidance and WTD threshold” section described below.