CCIE Cert Guide Notes — QoS Part III

  • LAN Switch Congestion Management and avoidance
    • 3550 and 3560 Switch Ingress Queuing
      • 3550 uses a single FIFO ingress queue as a place to hold frames, 3560 uses two ingress queues, one of which can be configured to be PQ.
      • 3560 uses Shared Round Robin (SRR) to control rates at which packets are sent. SRR shares the bandwidth among the queues according to the weights you configure.
      • Configuring ingress queuing:
        • allocate the ration by which to divide the ingress buffers to the two queues using: mls qos srr-queue input buffers %1 %2
        • mls qos srr-queue input bandwidth w1 w2: sets frequency at which the scheduler takes packets from the two queue.
        • to enable PQ, use: mls qos srr-queue input priority-queue queue-id bandwidth weight command. Weight parameter defines the %age of the link’s bandwidth that can be consumed by the PQ when there is competing traffic in the non-PQ.
        • ingress queues in 3560 use a method called Weighted Tail Drop (WTD) to set discard thresholds for each queue.
    • 3550 Egress Queuing
      • 3550 supports 4 queues per interface with classification based on CoS.
      • Scheduling is based on WRR with an optional expedited PQ.
      • makes most internal decisions based on internal DSCP setting.
        • the Frame’s internal DSCP is compared to a global DSCP-to-CoS map to determine a CoS value
        • the per-interface CoS-to-queue map determines the queue for a frame based on the assigned CoS.
      • wrr-queue bandwidth 10 20 30 40 and wrr-queue bandwidth 1 2 3 4 configure the SAME proportions.
      • 3550, egress queue FOUR (ONLY) on an interface as a PQ. priority-queue out is configured under the interface. WRR still applied on 1-3.
      • DSCP-toCoS map (upto 8 DSCPs can be mapped in a single command). e.g mls qos map dscp-cos 60 61 62 63 to 1
      • wrr-queue cos-map 3 6 7 (assigns cos 6 and 7 to queue 3).
      • sh mls qos int gi0/1 queue
    • 3560 Egress Queuing
      • improves by creating a mechanism to stop starvation due to PQ. Secondly, it adds shaping feature that slows down egress traffic.
      • same # of queues but only queue 1 can be PQ not queue 4.
      • 3560 has two options for scheduler: Shared Round Robin and Shaped Round Robin. Shared option also rate-limits (shapes) the queues so that they do not exceed the configured percentage of the link’s bandwidth.
      • Configuration:
        • srr-queue bandwidth share w1 w2 w3 w4
        • srr-queue bandwidth shape w1 w2 w3 w4
      • operation differs when queues are not full.
      • the shaped scheduler never allows any queue, PQ or non-PQ to exceed its configured percentage of link bandwidth, even if that means link sits idle.
  • 3550 Congestion Avoidance
    • Gig interfaces support, WRED or taildrop
    • FastE ports use switch specific method (not covered)
    • 3550 WRED has same strategy as WRED on routers but with many differences.
    • each egress queue has two WRED thresholds, which are $ages of the queue length.
    • thresholds can be set differently for each egress queue on each interface.
    • existence of single wrr-queue random-detect command under an interface both enables WRED on the interface for all queues and disabled default tail drop for each queue. If only one interface egress queue has wrr-queue random detect command, other three egress queues use WRED as well, however WRED defaults to use thresholds of 100%. To enable WRED-like behavior, each queue needs to have nondefault thresholds configured.
  • 3560 Congestion Avoidance
    • uses WTD (weighted Tail drop).
    • 3 thresholds per queue into which traffic can be divided based on CoS value.
    • WTD is configurable separately for all 6 queues in the 3560 (two ingress and four egress).


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