Help with QoS map for voice

[John Blakley]

All,

I have two sites on a P2P T1 connection that has been complaining about voice. The following policy map is applied on both ends of the link:

class-map match-any VOIP

match access-group 110

policy-map VOIP

class VOIP

priority 768

class class-default

fair-queue

random-detect dscp-based

Extended IP access list 110

permit ip any any dscp ef (5547188 matches)

permit ip any any precedence critical (77123 matches)

permit tcp any eq 1719 any (318 matches)

permit tcp any eq 1720 any (450 matches)

permit tcp any any eq 1719

permit tcp any any eq 1720

permit udp any eq 1719 any (1 match)

permit udp any eq 1720 any

permit udp any any eq 1719 (65 matches)

permit udp any any eq 1720 (8 matches)

I *think* that the priority 768 gives voice traffic 768k of bandwidth if there's data on the line. If there's no voice traffic, data can take the full T1 connection. Is there a better way to do this policy map? They've been complaining about static and jitter lately.

Thanks,

John

[Jon Marshall]

John

"I *think* that the priority 768 gives voice traffic 768k of bandwidth if there's data on the line."

Specifically, if there is congestion on the line then the voice data is guaranteed 768. If there is no congestion then the voice data can actually go over 768.

Are you sure that

1) You have captured all the relevant voice ports

2) You have allocated enough bandwidth for the number of calls that are being made.

Are you seeing congestion on the line ?

Jon

[John Blakley]

Jon,

This is an Avaya phone system. I'm not great with QoS stuff. (In fact, I'm still studying it.)

So, the phone subnet is the 10.10.0.0 network. I should fix my ACLs to reflect that as the source destination instead of "any."

How can I "know" if I've allocated enough bandwidth for the phones? Is there something that I can look for on the router?

Thanks!

John

[Jon Marshall]

John

Yes you should narrow it down as much as possible. Also as Edison says you are trusting all EF. You can do this but you must make sure that only you have marked traffic as such. So if you are relying on the end devices marking the traffic you need to be very careful as an end user could in theory mark all their traffic as EF.

As for working out how much bandwdith have a look at the link Edison has provided. You will need to know which codec is in use as this will have a subnstantial impact on amount of bandwidth needed.

Jon

[John Blakley]

I've been looking at the link. I'll need to get in touch with Avaya on the codec that it uses.

So, are there any show commands that I can see to try to get a good measurement on how much voice traffic I'm actually getting?

I guess my next question is:

If 768k is plenty of bandwidth, what could some other causes be on the static issue? Is there any other way that I could modify this policy to help with that?

Thanks,

John

[Jon Marshall]

Main thing would be to tie down the traffic so that you are sure only VOIP traffic gets into the priority queue.

Do you know where the marking of packets is taking place in your network ? Are you relying on the end user device or are you marking at the first switch ?

One other point. It may be worth posting into the VOIP forums as i'm sure they have seen this type of thing before.

Jon

[John Blakley]

It's more than likely done at the user end. The switches don't have qos configured on them, and the router is the only thing that's configured for qos. I don't know if marking is taking place at all actually.

I didn't set this up, but since they're starting to have problems, they want me to figure out how we can modify this. =)

John

[Jon Marshall]

John

"I don't know if marking is taking place at all actually"

It definitely is somewhere because of the matches in your acl's. Also you should really be considering end-to-end QOS for this setup. I am wary of introducing a whole lot of work for you but ideally you should be ensuring that only voice packets are marked with EF and that any data is marked or remarked at the ingress point to the network which is usually a switch.

No experience of Avaya system so can't really help there but there may be some people in the VOIP forums who have experience with it.

First things first, make sure your acl only references IP addresses that you know are tied to phones. Even this could be circumvented but it would be an improvement on where you are.

Do you have access to Avaya website because there should be docs on there that tell you which ports an Avaya VOIP system uses.

Jon

Jon

[Edison Ortiz]

You are blindly trusting any source with EF in their QoS Marking and placing on PQ?

I suggest tightening those ACLs a bit and just allocate PQ for voice traffic by including the source subnet from the voice network instead of using 'any'.

Do the same for the other entries.

You also need to calculate how many concurrent calls are made from that location so you can size the priority queue accordingly.

This link can be useful:

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml

HTH,

__

Edison.