VoIP QoS

Unanswered Question
Jun 16th, 2009

We have two VoIP phones (inter-tel) that connected to a 3550XL, the 3550XL is connected to another 3550 and that 3550 is connected to a 4510R which connects to the phone system. What is the best way to set up some sort of QoS on these switches to give priority to the phones on the far end?

thanks

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parshah Tue, 06/16/2009 - 10:20

Hi,

Thats quite a loaded question. However, the short of it is

1. Set the ports on the switches where phones and CM are connected to trust the DSCP/COS.

2. You might need to configure queuing so the switches handle the packets properly.

3. Make sure you don't overwrite the QoS markings, especially on the trunk ports.

Now, I know that is very vague. Try the following link, might be able to give you more info. Also, try auto qos on the 3550, it all in the link

http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.2_25_see/configuration/guide/swqos.html

Nicholas Matthews Tue, 06/16/2009 - 11:58

Remember this fact for QoS:

QoS only applies when there is contention for the bandwidth. Generally voice can handle handle delays up to 150 ms and jitter up to 20-30ms.

Most switched networks, at least what I've seen, don't have enough contention to really cause problems.

The places your Voice QoS becomes critical is on WAN interfaces and bottlenecks.

QoS on the actual port the phone is connected to is largely going to be unused. The only place to look are your trunks between switches. If you have packet loss on your trunks you likely have much larger problems than voice QoS.

-nick

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