Low-Latency Queueing With Strict Priority Queueing; Configured Accounting - Cisco ASR 9000 Series Configuration Manual

Low-Latency Queueing with Strict Priority Queueing

• Deficit counter—Number of bytes a queue has sent in each round. The counter is initialized to the
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 queuing (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.

Configured Accounting

Configured Accounting controls the overhead (packet length) for policing and shaping. The account option
can be specified with a service-policy when applying a policy to an interface. For bundle interfaces, the
configured accounting option is applied to all member interfaces.
The configured accounting option is available on ingress and egress policing, queuing and statistics for
CRS-MSC-140G. In CRS-MSC-40G, the configured accounting option is not available for queuing.
Prerequisites and Restrictions
• Allows packet size accounting tuning to match the QoS treatment provided at the connected interface.
• Supported on ASR 9000 Ethernet Linecards and Enhanced Ethernet Linecards.
• Supported accounting values are, from -48 to +48.
• Ingress shaping accounting is not supported (Ingress and egress policing accounting and egress shaping
• Dynamic changing of accounting overhead after application on policy is not supported
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide, Release
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quantum value.
accounting are supported).
Configuring Modular QoS Congestion Management