3560 QOS Notes

 

 

Both the ingress and egress queues are serviced by SRR, which controls the rate at which packets are sent. On the ingress queues, SRR sends packets to the internal ring. On the egress queues, SRR sends packets to the egress port. 

You can configure SRR on egress queues for sharing or for shaping. However, for ingress queues, sharing is the default mode, and it is the only mode supported.

In shaped mode, the egress queues are guaranteed a percentage of the bandwidth, and they are rate-limited to that amount. Shaped traffic does not use more than the allocated bandwidth even if the link is idle. Shaping provides a more even flow of traffic over time and reduces the peaks and valleys of bursty traffic. With shaping, the absolute value of each weight is used to compute the bandwidth available for the queues. 

In shared mode, the queues share the bandwidth among them according to the configured weights. The bandwidth is guaranteed at this level but not limited to it. For example, if a queue is empty and no longer requires a share of the link, the remaining queues can expand into the unused bandwidth and share it among them. With sharing, the ratio of the weights controls the frequency of dequeuing; the absolute values are meaningless. Shaping and sharing is configured per interface. Each interface can be uniquely configured.

Selective Packet Discard is the queue management technique for interface input queueing. The SPD commands are hidden in the IOS parser, but you can see them in the running configuration once you enter them. By default SPD is enabled in Normal mode. The following is the list of SPD commands:

spd enable
spd headroom 
spd extended-headroom 
ip spd mode aggressive
ip spd queue max-threshold 
ip spd queue min-threshold  

SPD input queuing (Selective Packet Discard) is desirable for a number of reasons. The first is for control plane security. It’s possible to block the router’s input queue with a high rate of malformed packets, which effectively blocks legitimate routing traffic. The result is a control plane DoS against the router. The next reason is for layer 2 keepalive, IGP, and BGP traffic separation.

Note the important fact that SPD thresholds are global for all queues. SPD computes Min and Max thresholds based on the lowest hold-queue size in the system. Therefore if you set the hold queue size lower on some interfaces, you will affect all other interface drop thresholds.