you take a look at IP precedence 5, you'll see that it is the same as a DSCP value of between 40 and 47. The decimal value 40 is 101000 in binary. The decimal value 47 is 101111 in binary. However, 48 is 110000 in binary-the first three bits change from 101 to 110, thus changing the IP precedence significant bits in the TOS field from 5 to 6. Table 31-1. Priority levels of different QoS types IP precedence Routing (Best Effort) Priority 16-23 Immediate 24-31 Flash 32-39 Flash-Override 40-47 Critical 48-55 Internet 56-63 Network Because the IP precedence values and the DSCP values are SO closely related, they can usually be interchanged. For example, if you're running a diffserv environment that uses DSCP values, but you have IP phones that will mark packets using only IP prece- dence values, you're still OK, because a diffserv value of 40 is the same bitwise as IP precedence value of 5. Flavors of QoS The following are the most commonly seen forms of QoS. If you're working with a VoIP network, you'll probably see CBWFQ on your routers and CoS on your switches: QoS Mechanics WFQ is the default queuing mechanism on 2 Mbps and slower serial links. For most implementations, the default configuration works fine. WFQ can be config- ured very specifically, but it usually isn't configured at all. When VoIP is involved, low-latency queuing is typically used in place of WFQ. Class-based weighted fair queuing Class-based weighted fair queuing (CBWFQ) allows you to configure classes of traffic and assign them to priorities and queues. CBWFQ is the basis for low-latency queuing. Priority queuing Priority queuing works just how it sounds: queues are created, and each class of packet is assigned to an appropriate queue based on priorities you design. Custom queuing Custom queuing is one of those features that you probably won't see much. You'll see it on Cisco exams, and you may see it on networks that had specific problems to be solved where voice was not a concern at the time of the resolution. That's not to say that custom q