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Network Warrior, 2nd Edition by Gary A. Donahue

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Troubleshooting a Converged Network

Dropping packets within specific queues can be hard to diagnose. After you’ve worked with a converged network for a while, though, you will get a feel for how the queues should behave. While every network is different, here are some common symptoms to look out for.

Incorrect Queue Configuration

When all the queues are configured correctly, none of them should drop packets when the link is congestion-free. When congestion occurs, packets should be dropped from the least important queue first. The least important queue is the class-default queue, which contains all nonprioritized packets.

Here, I have routed excessive traffic across our sample network’s T1 link. While most of the queues are busy, only the class-default queue is showing dropped packets:

R1#sho policy-map interface s0/1
 Serial0/1

  Service-policy output: WAN

    Class-map: Voice-RTP (match-any)
      11091627 packets, 2262021588 bytes
      5 minute offered rate 324000 bps, drop rate 0 bps
      Match: ip precedence 5
        11091627 packets, 2262021588 bytes
        5 minute rate 324000 bps
      Queueing
        Strict Priority
        Output Queue: Conversation 264
        Bandwidth 500 (kbps) Burst 12500 (Bytes)
        (pkts matched/bytes matched) 11091627/2262021588
        (total drops/bytes drops) 0/0

    Class-map: Voice-Control (match-all)
      7312459 packets, 1477128588 bytes
      5 minute offered rate 255000 bps, drop rate 0 bps Match: ip precedence 3 Queueing Output Queue: Conversation 265 Bandwidth 5 (%) Max Threshold 64 (packets) (pkts matched/bytes matched) 7312784/1477194238 ...

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