Queuing Tools 287

Example 4-7 CBWFQ—Classes for VoIP, NetMeeting, HTTP “important,” HTTP “not-so” Important, and

Everything Else (Continued)

max-reserved-bandwidth 85 service-policy output queue-on-dscp clockrate 128000

!

interface Serial0/0.1 point-to-point

description point-point subint global DLCI 103, connected via PVC to DLCI 101 ( R1)

ip address 192.168.2.253 255.255.255.0 frame-relay interface-dlci 101

!

! Portions omitted for brevity

!

access-list 101 permit udp host 192.168.3.100 range 16384 32767 192.168.1.0 0.0.0.255 range 16384 32767

!

The QoS policy requires the various queues to be assigned a total of 108 kbps of the 128 kbps of link bandwidth, which is about 84 percent of the link bandwidth. By default, CBWFQ only allows 75 percent of the link bandwidth to be explicitly assigned, reserving 25 percent for class class-default. Cisco recommends that you not change the setting, but you can with the max- reserved-bandwidth command, as shown in the configuration. By changing the max-reserved- bandwidth to 85 percent, the amounts of bandwidth that were suggested in the policy statements can be assigned. The danger is that the class-default, where important overhead such as keepalives and routing updates are queued, may not get enough bandwidth.

CBWFQ Summary

CBWFQ combines some of the best features of the various queuing tools into a single tool. Like CQ, CBWFQ can reserve a guaranteed amount of bandwidth per queue, but without the negative side effects of the CQ scheduler. CBWFQ can use WFQ as the default behavior for unclassified traffic. Packet loss behavior can take advantage of WRED, which reduces the possibilities of global synchronization. In addition, of all the queuing tools, CBWFQ has the largest variety of directly matchable fields for classifying packets.

CBWFQ does have some drawbacks, however. Most drawbacks are minor, but one negative is the lack of a PQ-like feature. Delayand jitter-sensitive traffic still suffers, even when enough bandwidth has been reserved by CBWFQ, because the CBWFQ scheduler can serve other queues when a VoIP or video packet is waiting in a queue. The next feature covered in the book, namely Low Latency Queuing (LLQ), overcomes this problem.

Table 4-13 summarizes some of the additional key comparison features among the WFQ family of queuing tools.