Intelligent Discards

Most 7705 SAR systems are susceptible to network processor congestion if the packet rate of small packets received on a node or card exceeds the processing capacity. If a node or card receives a high rate of small packet traffic, the node or card enters overload mode. Before the introduction of intelligent discards, when a node or card entered an overload state, the network processor would randomly drop packets.

The ‟intelligent discards during overload” feature allows the network processor to discard packets according to a preset priority order. In the egress direction, intelligent discards is applied to traffic entering the card from the fabric.

Traffic is discarded in the following order: low-priority out-of-profile user traffic is discarded first, followed by high-priority out-of-profile user traffic, then low-priority in-profile user traffic, high priority in-profile user traffic, and lastly control plane traffic. In the ingress direction, intelligent discards is applied to traffic entering the card from the physical ports. Traffic is discarded in the following order: low-priority user traffic is always discarded first, followed by high-priority user traffic. This order ensures that low-priority user traffic is always the most susceptible to discards.

In the egress direction, the system differentiates between high-priority and low-priority user traffic based on the internal forwarding class and queue-type fabric header markers. In the ingress direction, the system differentiates between high-priority and low-priority user traffic based on packet header bits. Table: Ingress Packet Priority During Overload details the classification of user traffic in the ingress direction.

Table: Ingress Packet Priority During Overload

Fabric Header Marker

High-priority Values

Low-priority Values

MPLS TC

7 to 4

3 to 0

IP DSCP

63 to 32

31 to 0

Eth Dot1p

7 to 4

3 to 0

Intelligent discards during overload ensures priority-based handling of traffic and helps existing traffic management implementations. It does not change how QoS-based classification, buffer management, or scheduling operates on the 7705 SAR. If the node or card is not in overload operation mode, there is no change to the way packets are handled by the network processor.

There are no commands to configure intelligent discards during overload; the feature is automatically enabled on the following cards, modules, and ports:

Parent topic: QoS Overview