Information Technology Reference
In-Depth Information
Cisco IP phones rather than forcing the switches to apply QoS markings to a higher vol-
ume of traffic.
If you have PCs attached to the network and you have access layer switches with QoS ca-
pabilities, you can begin marking at these devices (point 2 in Figure 6-20). If your access
layer switches do not have QoS capabilities, then the first possible place you can apply
QoS markings is at the distribution layer switches (point 3 in Figure 6-20). This will work
just fine; however, it adds an extra load to the distribution layer switches. Likewise, you
will have network traffic passing through access layer switches without any QoS treat-
ment. Although this is usually a safe bet—because access layer switches typically have
higher-speed connections, on which congestion is rare—it is always best to apply QoS in
as many places as possible where there is a potential bottleneck.
Note:
AutoQoS uses Cisco Discovery Protocol (CDP) to detect Cisco IP phones on
Cisco switches and properly configure the QoS settings. This ensures that a user cannot
disconnect their IP phone and attach another device to receive high-priority network treat-
ment. Be sure you do not disable CDP on switches supporting Cisco IP phones.
Now, we have come to the point of configuring AutoQoS. Amazingly, by Cisco's design,
enabling AutoQoS is accomplished through a single command applied under interface
configuration mode. To enable AutoQoS in your network, you must first identify the in-
terfaces to which applying AutoQoS makes sense. AutoQoS does not need to be applied
under every switch and router interface in your network (although it probably won't hurt
anything if you did this). It primarily should be applied to interfaces on which the devices
or applications need special or preferred treatment over others. Figure 6-21 shows a typi-
cal network. The interfaces labeled A represent areas of the network where you would use
AutoQoS.
As you can see from Figure 6-21, you'll be typing this one command many times. Before
you enter the AutoQoS command, always ensure that you have entered the correct band-
width statement under the serial interfaces of your routers, because a router cannot auto-
detect the actual speed of a WAN connection. A router can detect all other interfaces
without requiring the bandwidth command.
Note:
AutoQoS uses a sophisticated queuing method known as Low Latency Queuing
(LLQ). This queuing method provisions a specific amount of bandwidth for the various
types of network traffic, including voice. Using AutoQoS features with incorrectly config-
ured bandwidth commands can cause substandard network service.
The AutoQoS command syntax might be slightly different depending on where you enter
it. The syntax in Example 6-28 enables AutoQoS for the interfaces shown in Figure 6-21
that are connected to the Cisco IP phones.
