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Understanding CoS on ATM IMA Pseudowire Interfaces Overview


ACX Series routers configured with Asynchronous Transfer Mode (ATM) inverse multiplexing for ATM (IMA) pseudowire interfaces support class of service (CoS) features for ingress and egress traffic. Policing is performed by monitoring the configured parameters on incoming traffic to conserve resources by dropping traffic that might not meet those configured parameters. Egress shaping uses queuing and scheduling to control the bandwidth used. Fixed classification is provided per interface.


ACX5048 and ACX5096 routers do not support ATM IMA pseudowire configurations.

ATM IMA pseudowires with the following encapsulation are supported:

  • atm-ccc-cell-relay

  • atm-ccc-vc-mux

The following ATM IMA CoS features are supported:

Cell-Based ATM Policing

Policing, or rate limiting, enables you to limit the amount of traffic that passes into or out of the interface. Policing works with firewall filters to thwart denial-of-service (DoS) attacks. Networks police traffic by limiting the input or output transmission rate of a class of traffic on the basis of user-defined criteria. The ATM policer controls the maximum rate of traffic sent from or received on the interface on which it is applied. To apply limits to the traffic flow, configure the cdvt and peak-rate parameters within the policer. Define the policing-action parameter as discard, discard-tag, or count to set a consequence for the packets that exceed these limits. The consequence of configuring the discard-tag statement is usually a higher loss priority so that if those packets encounter downstream congestion, they are discarded first.

On ACX Series routers, policing is cell based and configured in the ingress path of the ATM IMA pseudowire interface at the [edit firewall] hierarchy level. The following ATM policing features are supported:

  • ATM Adaption Layer 5 (AAL5) pseudowires on which cell-based policing is performed before packet assembly.

  • Per-ATM IMA channel policing.

  • Traffic classes—Constant bit rate (cbr), real-time variable bit rate (rtvbr), non-real-time variable bit rate (nrtvbr), and unspecified bit rate (ubr). All traffic classes must include the peak-rate and cdvt statements for the configuration to work. With the peak-rate statement, you can limit the maximum traffic allowed by specifying the largest number of cells per second that the policer processes before it drops packets. The cdvt statement ensures that the configuration functions correctly.

  • For nonconforming cells, the discard, discard-tag, and count actions at the [edit firewall atm-policer policer-name] hierarchy level. The discard-tag action is applicable to variable bit-rate—nrtvbr and rtvbr—traffic classes.

Cell-Based ATM Shaping

Cell-based ATM shaping uses cell-based queuing and scheduling to determine the maximum amount of traffic that can be transmitted on an ATM IMA pseudowire. Packet-based shaping is not supported. On ACX Series routers, ATM shaping is configured in the egress path of the ATM IMA pseudowire interface at the [edit class-of-service] hierarchy level. The following ATM shaping features are supported:

  • Prioritized bit rate—Constant bit rate (cbr) is the highest priority, followed by variable bit rate—nrtvbr and rtvbr. Unspecified bit rate (ubr) is similar to the best-effort service for Ethernet traffic.

  • Constant bit rate shaping—Constant bit rate (cbr) shaping uses the peak cell rate to limit the number of cells per second that the shaper processes before it drops packets.

  • Variable bit rate shaping—Variable bit rate shaping (nrtvbr and rtvbr) uses peak-rate and sustained-rate.

  • Unspecified bit rate—Unspecified bit rate (ubr) uses peak-rate with the lowest transmit priority.

The default shaping parameter is unspecified bit rate, which is similar to the best-effort service for Ethernet traffic.

Fixed Classification

Fixed classifiers map all traffic on an interface to the forwarding class and loss priority. The forwarding class determines the output queue. A scheduler uses the loss priority to control packet discard during periods of congestion by associating different drop profiles with different loss priorities. On ACX Series routers, the fixed classifier is associated with the ingress interface. Packets are assigned on the basis of the type of fixed classification associated with the logical interface. To configure a fixed classifier, include the forwarding-class class-name statement at the [edit class-of-service interface interface-name unit logical-unit-number hierarchy level.