Cisco 1721s point-to-point VOIP QoS

jimcmu used Ask the Experts™
My office has two locations, Cisco 1721s are used to connect a full point-to-point T1 between the two offices.  There is a separate Cisco 1721 at the main site that is connected to the internet, so all traffic from the remote office is routed to the main site at this time.

We recently purchased an NEC Aspire VOIP system for the organization.  The remote site has approximately 7 VOIP phones and 7 workstations.  Occasionally, we would experience choppy conversations and low volume when speaking with users in the remote office.

My question is, how would I implement QoS on the Cisco 1721s to prioritize voice traffic?  Any sample configs would be greatly appreciated.

Thanks in advance!
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Top Expert 2007
You can reserve a certain amount of bandwidth depending on your protocol to make sure that voice and signaling are prioritized.

I'm not 100% sure how the aspire marks outbound traffic but normally voice is ef and signaling is af31. You can adjust to fit your needs and setup.

class-map match-any ef
 match ip dscp cs3  af31  cs5  ef

policy-map Voice
 class ef
  priority 307
  set dscp ef
 class class-default

Interface s0
ip address
service-policy output Voice

Hope this helps
You will also need to ensure that qos is configured on all network devices between the phones and the call processing system, i.e. switches etc.

I would recommend creating two class maps, one for ef and one for af31. ef is used by the voice stream iteslf, and af31 is usually used for voice signalling.

so, I would recommend the config below:-

class-map match-any VoipSig
 match ip dscp af31
class-map match-any voip
 match protocol rtp audio
 match  dscp ef
policy-map Voice
 class voip
  priority percent 70
  set dscp ef
 class VoipSig
  bandwidth remaining percent 40
  set dscp cs3

Interface s0
ip address
service-policy output Voice
 ip nbar protocol-discovery

The last line here allows the router to detect what protocol different packets are using and match them to rules. If you know what signalling protocol your phone system is using, you can configure it using the match protocol command. Also, if all your voice traffic is coming from one subnet, you can create an access list and use a match command to match that to a class map.

This policy will allow voice traffic to take up to 70% of the overall bandwith and still be prioritised. Voice control traffic can take up to 40% of the remaining bandwith. You can of course tweak these values based on your environment.

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