QoS is covered in detail in Chapter 31, and it’s a topic that could easily fill an entire book. In this section, I will focus on some 3750-specific QoS features.

One of the useful features of the 3750 is support for AutoQoS, which makes certain assumptions about your network and configures the switch accordingly. While I’m not a fan of letting network devices assume anything, in this case, the assumptions are accurate most of the time. I have had no qualms about enabling AutoQoS on the 3750s I’ve installed in VoIP networks with hundreds of phones supported by Cisco Call Manager. The reason I’m OK with this is that Cisco’s assumptions are built around the idea that you’re using Call Manager, Cisco IP phones, and low-latency queuing on your network. Chances are, if you need QoS enabled on your switches, it’s because you’re implementing VoIP.

You can enable AutoQoS on an interface with the auto qos voip command:

3750(config-if)#auto qos voip ?
  cisco-phone      Trust the QoS marking of Cisco IP Phone
  cisco-softphone  Trust the QoS marking of Cisco IP SoftPhone
  trust            Trust the DSCP/CoS marking

There are three options: cisco-phone, cisco-softphone, and trust. The first two are used for interfaces connected to either hard- or softphones. When configured with these options, the QoS values received in packets will be trusted only if they’re sourced from Cisco IP phones. The trust option enables QoS while trusting all packets’ QoS values.

If you’d like to see what AutoQoS does, enable AutoQoS debugging ...

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