features allow you to control congestion by determining the order in which a
traffic flow (or packets) is sent out an interface based on priorities assigned
to packets. Congestion management entails the creation of queues, assignment of
packets to those queues based on the classification of the packet, and
scheduling of the packets in a queue for transmission. The congestion
management features in
Cisco IOS XR software
allow you to specify creation of a different number of queues, affording
greater or lesser degree of differentiation of traffic, and to specify the
order in which that traffic is sent.
During periods with light
traffic flow, that is, when no congestion exists, packets are sent out the
interface as soon as they arrive. During periods of transmit congestion at the
outgoing interface, packets arrive faster than the interface can send them. If
you use congestion management features, packets accumulating at an interface
are queued until the interface is free to send them; they are then scheduled
for transmission according to their assigned priority and the queuing method
configured for the interface. The router determines the order of packet
transmission by controlling which packets are placed in which queue and how
queues are serviced with respect to each other.
In addition to queuing
methods, QoS congestion management mechanisms, such as policers and shapers,
are needed to ensure that a packet adheres to a contract and service. Both
policing and shaping mechanisms use the traffic descriptor for a packet.
Policers and shapers usually
identify traffic descriptor violations in an identical manner through the token
bucket mechanism, but they differ in the way they respond to violations. A
policer typically drops traffic flow; whereas, a shaper delays excess traffic
flow using a buffer, or queuing mechanism, to hold the traffic for transmission
at a later time.
Traffic shaping and policing
can work in tandem. For example, a good traffic shaping scheme should make it
easy for nodes inside the network to detect abnormal flows.
For Clear Channel ATM SPAs, all queue-based actions are
offloaded to the SPA and are performed by the SPA. Clear Channel ATM
subinterfaces support eight queues per subinterface. On egress subinterfaces,
you can configure a service policy with a maximum of seven non-default classes
with queueing actions. Other classes must
not have queueing