Perform this task to establish the match criteria (or mark the packets) to be used to match packets for classifying network traffic.

The following example shows how to use the match precedence command to manage IPv6 traffic flows:

Router# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
 Router(config)# class-m c1
  Router(config-cmap)# match precedence 5
  Router(config-cmap)# end
Router#
 Router(config)# policy p1
  Router(config-pmap)# class c1
  Router(config-pmap-c)# police 10000 conform set-prec-trans 4

To verify that packet marking is working as expected, use the show policy command. The output of this command shows a difference in the number of total packets versus the number of packets marked.

Router# show policy p1
  Policy Map p1
    Class c1
      police 10000 1500 1500 conform-action set-prec-transmit 4 exceed-action drop
Router# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)# interface serial 4/1
Router(config-if)# service out p1
Router(config-if)# end
Router# show policy interface s4/1
 Serial4/1 
  Service-policy output: p1
    Class-map: c1 (match-all)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: precedence 5 
      police:
        10000 bps, 1500 limit, 1500 extended limit
        conformed 0 packets, 0 bytes; action: set-prec-transmit 4
        exceeded 0 packets, 0 bytes; action: drop
        conformed 0 bps, exceed 0 bps violate 0 bps
    Class-map: class-default (match-any)
      10 packets, 1486 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: any 

During periods of transmit congestion at the outgoing interface, packets arrive faster than the interface can send them. It is helpful to know how to interpret the output of the show policy-map interface command, which is useful for monitoring the results of a service policy created with Cisco’s MQC.

Congestion typically occurs when a fast ingress interface feeds a relatively slow egress interface. Functionally, congestion is defined as filling the transmit ring on the interface (a ring is a special buffer control structure). Every interface supports a pair of rings: a receive ring for receiving packets and a transmit ring for sending packets. The size of the rings varies with the interface controller and with the bandwidth of the interface or virtual circuit (VC). As in the following example, use the show atm vc vcd command to display the value of the transmit ring on a PA-A3 ATM port adapter.

Router# show atm vc 3
 
ATM5/0.2: VCD: 3, VPI: 2, VCI: 2 
VBR-NRT, PeakRate: 30000, Average Rate: 20000, Burst Cells: 94 
AAL5-LLC/SNAP, etype:0x0, Flags: 0x20, VCmode: 0x0 
OAM frequency: 0 second(s) 
PA TxRingLimit: 10 
InARP frequency: 15 minutes(s) 
Transmit priority 2 
InPkts: 0, OutPkts: 0, InBytes: 0, OutBytes: 0 
InPRoc: 0, OutPRoc: 0 
InFast: 0, OutFast: 0, InAS: 0, OutAS: 0 
InPktDrops: 0, OutPktDrops: 0 
CrcErrors: 0, SarTimeOuts: 0, OverSizedSDUs: 0 
OAM cells received: 0 
OAM cells sent: 0 
Status: UP 

Cisco software (also referred to as the Layer 3 processor) and the interface driver use the transmit ring when moving packets to the physical media. The two processors collaborate in the following way:

• The interface sends packets according to the interface rate or a shaped rate.

• The interface maintains a hardware queue or transmit ring, where it stores the packets waiting for transmission onto the physical wire.

• When the hardware queue or transmit ring fills, the interface provides explicit back pressure to the Layer 3 processor system. It notifies the Layer 3 processor to stop dequeuing packets to the interface’s transmit ring because the transmit ring is full. The Layer 3 processor now stores the excess packets in the Layer 3 queues.

• When the interface sends the packets on the transmit ring and empties the ring, it once again has sufficient buffers available to store the packets. It releases the back pressure, and the Layer 3 processor dequeues new packets to the interface.

The most important aspect of this communication system is that the interface recognizes that its transmit ring is full and throttles the receipt of new packets from the Layer 3 processor system. Thus, when the interface is congested, the drop decision is moved from a random, last-in, first-dropped decision in the first in, first out (FIFO) queue of the transmit ring to a differentiated decision based on IP-level service policies implemented by the Layer 3 processor.

Service policies apply only to packets stored in the Layer 3 queues. The table below illustrates which packets sit in the Layer 3 queue. Locally generated packets are always process switched and are delivered first to the Layer 3 queue before being passed on to the interface driver. Fast-switched and Cisco Express Forwarding-switched packets are delivered directly to the transmit ring and sit in the L3 queue only when the transmit ring is full.

The following example shows these guidelines applied to the show policy-map interface command output.

Router# show policy-map interface atm 1/0.1
 
ATM1/0.1: VC 0/100 - 
 Service-policy output: cbwfq (1283) 
   Class-map: A (match-all) (1285/2) 
     28621 packets, 7098008 bytes
 
     5 minute offered rate 10000 bps, drop rate 0 bps 
     Match: access-group 101 (1289) 
     Weighted Fair Queueing 
       Output Queue: Conversation 73 
       Bandwidth 500 (kbps) Max Threshold 64 (packets) 
       (pkts matched/bytes matched) 28621/7098008
 
       (depth/total drops/no-buffer drops) 0/0/0
   Class-map: B (match-all) (1301/4)
 
     2058 packets, 148176 bytes 
     5 minute offered rate 0 bps, drop rate 0 bps 
     Match: access-group 103 (1305) 
     Weighted Fair Queueing 
       Output Queue: Conversation 75 
       Bandwidth 50 (kbps) Max Threshold 64 (packets) 
       (pkts matched/bytes matched) 0/0 
       (depth/total drops/no-buffer drops) 0/0/0 
   Class-map: class-default (match-any) (1309/0) 
     19 packets, 968 bytes 
     5 minute offered rate 0 bps, drop rate 0 bps 
     Match: any  (1313)

The table below defines counters that appear in the example.

Without congestion, there is no need to queue any excess packets. When congestion occurs, packets, including Cisco Express Forwarding- and fast-switched packets, might go into the Layer 3 queue. If you use congestion management features, packets accumulating at an interface are queued until the interface is free to send them; they are then scheduled according to their assigned priority and the queueing mechanism configured for the interface.

Normally, the packets counter is much larger than the packets matched counter. If the values of the two counters are nearly equal, then the interface is receiving a large number of process-switched packets or is heavily congested. Both of these conditions should be investigated to ensure optimal packet forwarding.

Routers allocate conversation numbers for the queues that are created when the service policy is applied. The following example shows the queues and related information.

Router# show policy-map interface s1/0.1 dlci 100
 
      Serial1/0.1: DLCI 100 - 
      output : mypolicy 
       Class voice 
        Weighted Fair Queueing 
            Strict Priority 
            Output Queue: Conversation 72
 
              Bandwidth 16 (kbps) Packets Matched 0 
             (pkts discards/bytes discards) 0/0 
       Class immediate-data 
        Weighted Fair Queueing 
            Output Queue: Conversation 73
 
              Bandwidth 60 (%) Packets Matched 0 
              (pkts discards/bytes discards/tail drops) 0/0/0 
              mean queue depth: 0 
              drops: class  random   tail     min-th   max-th   mark-prob 
                     0      0        0        64       128      1/10 
                     1      0        0        71       128      1/10 
                     2      0        0        78       128      1/10 
                     3      0        0        85       128      1/10 
                     4      0        0        92       128      1/10 
                     5      0        0        99       128      1/10 
                     6      0        0        106      128      1/10 
                     7      0        0        113      128      1/10 
                     rsvp   0        0        120      128      1/10 
       Class priority-data 
        Weighted Fair Queueing 
            Output Queue: Conversation 74
 
              Bandwidth 40 (%) Packets Matched 0 Max Threshold 64 (packets) 
              (pkts discards/bytes discards/tail drops) 0/0/0 
       Class class-default 
        Weighted Fair Queueing 
            Flow Based Fair Queueing 
            Maximum Number of Hashed Queues 64  Max Threshold 20 (packets)

Information reported for each class includes the following:

• Class definition

• Queueing method applied

• Output Queue Conversation number

• Bandwidth used

• Number of packets discarded

• Number of bytes discarded

• Number of packets dropped

The class-defaultclass is the default class to which traffic is directed, if that traffic does not satisfy the match criteria of other classes whose policy is defined in the policy map. The fair-queue command allows you to specify the number of dynamic queues into which IP flows are sorted and classified. Alternately, routers allocate a default number of queues derived from the bandwidth on the interface or VC. Supported values in either case are a power of two, in a range from 16 to 4096.

The table below lists the default values for interfaces and for ATM permanent virtual circuits (PVCs).

The table below lists the default number of dynamic queues in relation to ATM PVC bandwidth.

Based on the number of reserved queues for WFQ, Cisco software assigns a conversation or queue number as shown in the table below.