QoS dilemma

Kevin Dorrell · ‎03-05-2008

We are installing IP Phones and PBX from a vendor that is not Cisco. I have a problem with QoS.

The phones are marking voice with CoS 5 and DSCP EF, and signalling with CoS 3 and DSCP AF31. So I have set all my priority queues to handle the CoS 5, and a different queue for CoS 3. All normal so far.

But the PBX has a gateway card to connect the IP phone world with the POTS world. This gateway card is marking voice traffic with CoS 6 and DSCP CS6, and signalling with CoS 5 and DSCP CS5. Is this normal?

It messes up my QoS scheme, because we now have voice from the phones that has the same CoS as signalling from the gateway card. And the voice from the gateway card is going into a queue that takes its chances with any other CoS 4, 6, and 7 traffic.

At the root of the problem is that this gateway card is connected to a 2950-SX series switch. That switch has a common queueing policy for all its interfaces you cannot have a different policy from the for phones-to-gateway than for gateway-to-phones.

Has anyone else seen this, and can someone suggest a way out? AFAIK, there is no CoS or DSCP re-write on a 2950.

The only thing I can think of is to put both CoS 5 and CoS 6 into the expedited queue. That means the gateway's signalling will be expedited, which I don't really want - I want it at AF31 and queued with the rest of the CoS 3 traffic.

Someone with more experience ... please help!

Kevin Dorrell

Luxembourg

Joseph W. Doherty · ‎03-05-2008

I'm guessing the PBX can't reconfigure how it marks? (Some can, but CoS and DSCPs are often new concepts to TelCo people. I.e. you sometimes have to ask the PBX support person to look for such settings or ask the vendor.)

If not, no possibility of placing a smarter switch between the PBX and the existing 2950?

If still no, perhaps live with the combined incorrect markings within just that switch, but reclassify at the first opportunity.

Kevin Dorrell · ‎03-06-2008

Thanks Joseph.

So far, the vendor's technician has said that the only option available is layer-2 marking on/off, but I shall ask him to check again.

As you suggest, I could put a smarter switch between the PBX and the 2950. I have a nice new 3560-8PC that I could use for that, but it seems a bit of a waste to use it just for re-marking traffic when the PBX should have done it correctly in the first place.

And, yes, I could simply put up with it for the first hop, and then re-classify once it is uplinked into the 4500, so long as the 4500 is capable of classifying and marking according to source IP address and existing CoS.

Kevin Dorrell

Luxembourg