Troubleshoot and understand QoS on N5k

 Ok pete today you had a with INE, and during this lab you noticed that when you where doing Jumbo MTU as per his request, you said OK all traffic for CoS 4 get's a jumbo MTU, but he had a MDS connected that was providing the iSCSI, and this was not getting jumbo frames or not supporting them.

Here is a way to make it actually work:



interface Ethernet1/4
  untagged cos 4
  switchport mode trunk

Because based on your policy, on CoS 4 was allowed jumbo frames, so this was going to cause issues, so you used the show queuing command to verify this worked, DO NOT UNDERSTIMATE THE SHOW QUEUING COMMAND





N5K8(config-if)# show queuing int eth1/14
Ethernet1/14 queuing information:
  TX Queuing
    qos-group  sched-type  oper-bandwidth
        0       WRR             50
        1       WRR             50
        4       WRR              0

  RX Queuing
    qos-group 0
    q-size: 143360, HW MTU: 1600 (1500 configured)
    drop-type: drop, xon: 0, xoff: 143360
    Statistics:
        Pkts received over the port             : 157382
        Ucast pkts sent to the cross-bar        : 157241
        Mcast pkts sent to the cross-bar        : 141
        Ucast pkts received from the cross-bar  : 93
        Pkts sent to the port                   : 117
        Pkts discarded on ingress               : 0
        Per-priority-pause status               : Rx (Inactive), Tx (Inactive)

    qos-group 1
    q-size: 76800, HW MTU: 2240 (2158 configured)
    drop-type: no-drop, xon: 20480, xoff: 38400
    Statistics:
        Pkts received over the port             : 0
        Ucast pkts sent to the cross-bar        : 0
        Mcast pkts sent to the cross-bar        : 0
        Ucast pkts received from the cross-bar  : 0
        Pkts sent to the port                   : 0
        Pkts discarded on ingress               : 0
        Per-priority-pause status               : Rx (Inactive), Tx (Inactive)

    qos-group 4
    q-size: 81920, HW MTU: 9280 (9216 configured)
    drop-type: no-drop, xon: 20480, xoff: 36800
    Statistics:
        Pkts received over the port             : 38718
        Ucast pkts sent to the cross-bar        : 38697
        Mcast pkts sent to the cross-bar        : 21
        Ucast pkts received from the cross-bar  : 99742
        Pkts sent to the port                   : 99813
        Pkts discarded on ingress               : 0
        Per-priority-pause status               : Rx (Inactive), Tx (Inactive)

  Total Multicast crossbar statistics:
    Mcast pkts received from the cross-bar      : 95



So here is a few things about QoS on the 5k that you might have missed:


- ALL INTERFACES ARE TRUSTED BY DEFAULT
- ALL TRAFFIC IS MARKED WITH A COS VALUE AS IT COMES IN, EVEN UNTAGGED TRAFFIC, BUT AS SHOWN ABOVE YOU CAN SPECIFY WHAT COS YOU WANT TO USE
FOR UNTAGGED
- WHEN YOU "MULTICAST-OPTIMIZE" A CLASS, (WHICH YOU CAN ONLY DO FOR ONE CLASS IN THE POLICY-MAP) YOU ARE SAYING THAT THIS CLASS IS ALLOWED TO REUSE ALL THE MULTICAST QUEUES NOT USED BY OTHER CLASSES (THERE ARE 128 OF THESE ON AN N5K!)