Need Help Implement QoS

Unanswered Question
Aug 23rd, 2007

I want to implement QoS to improve voice quality on our network. We are a Cisco shop. The switches are 2950's and 3750's and the routers used ranges from 2800's to 3845's. Should I implement AutoQoS or configure QoS manually? If Autoqos is suggested, what IOS on the switches and routers supports this? Can you send sample configs? I'm new to QoS. TIA.

David

I have this problem too.
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Joseph W. Doherty Thu, 08/23/2007 - 11:41

Cisco's configuration guides and whitepapers are excellent and worth reading. E.g. http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_guide_chapter09186a008081df7e.html They will do a much better job than I could in a short response, so I'll limit myself to a few points.

AutoQoS or not? Well, the idea is to avoid the need to build and maintain your own QoS model. In principal, fine idea. Believe there are different flavors of AutoQoS, but believe all will prioritize voice. Personally my QoS experience predates AutoQoS, so I'm biased, and I find the later variants overly complex, at least on routers. Believe you can customize what they generate, but if your willing to do that, why not make you own (doesn't have to be complex for routers - see below).

Usually, all you need to do in insure the voice traffic is expedited, without drops, and its control traffic isn't dropped. Assuming your voice packets have a marking of DSCP EF, something as simple as this on your WAN routers often works well:

(syntax might be off - also syntax various between some IOS versions)

class-map Voice

match ip DSCP EF (assuming phones tag the packet, or you'll need to tag it)

policy CBWFQ

class Voice

priority percent 50

class class-default

fair-queue

interface serial 0

service policy outbound CBWFQ

The above places voice traffic in what's known as LLQ (low latency queue), if present it goes first. Everything else alternates between all the flows.

On 2950 or 3750, service policies for outbound are not supported, but you can accomplish something functionally similar except the FQ for everything else, above, isn't available. On those platforms you might need to place the voice control traffic in it's own outbound class. Here is where you might want to try AutoQoS.

Since WAN links usually have less bandwidth, and higher latency, QoS is often critical. For LAN links, there often is so much excess bandwidth that you won't see sustained congestion, or nothing but short term transient congestion. If the latter is true in your environment, you can get by without QoS, but at risk.

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