Packet classification and marking techniques identify the traffic flow, and provide the capability to partition network traffic into multiple priority levels or classes of service. After classification is complete, any other QoS actions can be performed.

Identification of a traffic flow can be performed by using several methods within a single router, such as access control lists (ACLs), protocol match, IP precedence, IP differentiated service code point (DSCP), MPLS EXP bit, or Class of Service (CoS).

Marking of a traffic flow is performed by:

• Setting IP Precedence or DSCP bits in the IP Type of Service (ToS) byte.

• Setting (CoS, DEI) bits in the Layer 2 headers.

• Setting EXP bits within the imposed or the topmost Multiprotocol Label Switching (MPLS) label.

• Setting qos-group and discard-class bits.

Marking can be carried out:

• Unconditionally—As part of the class-action.

• Conditionally—As part of a policer-action.

• Combination of conditionally and unconditionally.

You can set a maximum of two fields per class-map. For example:

class c1
set cos 2
set dscp 21

You can place these two fields in any of the following combinations:

• Two sets per policer (conform/exceed/violate)

• Two sets without policing

For example:

Policy-map p1
class c1
     set dscp af11
     set cos 5
     police rate 10 mbps peak-rate 20 mbps
          conform-action set dscp af12
          conform-action set cos 6
          exceed-action set dscp af13
          exceed-action set cos 4

For detailed conceptual and configuration information about packet marking, see the “Configuring Modular Quality of Service Packet Classification on Cisco ASR 9000 Series Routers” module in this guide for unconditional marking, and the “Configuring Modular Quality of Service Congestion Management on Cisco ASR 9000 Series Routers” module in this guide for conditional marking.

Congestion management techniques control congestion after it has occurred. One way that network elements handle an overflow of arriving traffic is to use a queuing algorithm to sort the traffic, then determine some servicing method of prioritizing it onto an output link.

Cisco IOS XR software implements the low-latency Queuing (LLQ) feature, which brings strict priority queuing (PQ) to the Modified Deficit Round Robin (MDRR) scheduling mechanism. LLQ with strict PQ allows delay-sensitive data such as voice, to be dequeued and sent before packets in other queues are dequeued.

Cisco IOS XR software includes traffic policing capabilities available on a per-class basis as well as class-based shaping.

The traffic policing feature limits the input or output transmission rate of a class of traffic based on user-defined criteria, and can mark packets by setting values such as IP Precedence, QoS group, or DSCP value.

Traffic shaping allows control over the traffic that leaves an interface to match its flow to the speed of the remote target interface and ensure that the traffic conforms to the policies contracted for it. Thus, traffic adhering to a particular profile can be shaped to meet downstream requirements, thereby eliminating bottlenecks in topologies with data-rate mismatches.

Cisco IOS XR software supports a class-based traffic shaping method through a CLI mechanism in which parameters are applied per class.

For detailed conceptual and configuration information about congestion management, see the “Configuring Modular Quality of Service Congestion Management on Cisco ASR 9000 Series  Routers” module.

Congestion avoidance techniques monitor network traffic flows in an effort to anticipate and avoid congestion at common network and internetwork bottlenecks before problems occur. These techniques are designed to provide preferential treatment for traffic (such as a video stream) that has been classified as real-time critical under congestion situations while concurrently maximizing network throughput and capacity utilization and minimizing packet loss and delay. Cisco IOS XR software supports the Random Early Detection (RED), Weighted RED (WRED), and tail drop QoS congestion avoidance features.

For detailed conceptual and configuration information about congestion avoidance techniques, see the “Configuring Modular Quality of Service Congestion Management on Cisco ASR 9000 Series Routers” module in this guide.