Posts from ‘QoS’


In this first of a series of blog posts regarding Catalyst QoS, we will exam the AutoQoS capabilities on the 3560 Catalyst devices. AutoQoS allows for the automation of QoS settings for the switch with an absolute minimum of configuration required from the engineer. In particular, the 3560 AutoQoS features automates the classification and congestion management configurations required in VoIP environments. You should note that the 3560 AutoQoS has much “catching up” to do when you compare this feature to AutoQoS for VoIP and AutoQoS for Enterprise that are both now possible in the pure router class of Cisco devices.

First, the easy part. The interface configuration command required for QoS is simply:

auto qos voip [cisco-phone | cisco-softphone | trust]

Notice the auto qos voip command is used in conjunction with keywords that specify what devices to “trust” when it comes to these important VoIP packets. The cisco-phone keyword instructs the AutoQoS feature to only trust and act upon the incoming voice packets if they are truly sent from a Cisco IP Phone. The phone’s presence is detected thanks to CDP. Similarly, the cisco-softphone keyword instructs the device to only trust and act upon the voice packets if they are sent from a Cisco phone running in software on a PC. Finally, the trust keyword instructs the device to trust markings for VoIP packets that are coming from another switch or router over the port.

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Many people have problems understanding the meaning of Bc (committed burst) used with traffic policing. Everyone seems to know the “magic” formula (Bc=1,5sec*CIR) but have a vague understanding of the reasons behind it. Let’s clear the confusion and see what Bc really affects when it comes to policing.

Averaging and Smoothing

Imagine you’re driving a car and want to find out your speed. In order to do this, you need to count the time (T) it takes you to pass the distance (S). The speed is then V=S/T – what a nice looking elementary school formula. So if you drove 100 miles in 1 hour your speed is 100 Mph. However, if you drove 50 miles in 30 minutes, your speed is the same 100 Mph. The only difference between the two measurements is the time interval used. Ideally, the only real value is your instant speed defined as the limit of S/T with T going to zero. However, this only works well in mathematics – in the real world, you always need a finite time interval to perform the measurement.
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QoS features available on Catalyst switch platforms have specific limitations, dictated by the hardware design of modern L3 switches, which is heavily optimized to handle packets at very high rates. Catalyst switch QoS is implemented using TCAM (Ternary Content Addressable Tables) – fast hardware lookup tables – to store all QoS configurations and settings. We start out Catalyst QoS overview with the old, long time available in the CCIE lab, the Catalyst 3550 model.

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