QoS Config Help

[mbalik]

Hello,

I am beginning to configure QoS to give priority to a non-Cisco VoIP phone system. I am keeping the configuration simple. I want to give priority to the voice traffic and all other network traffic can be left as using best effort.

I know that I must first classify the traffic. My voice VLANs use various subnets all included within the larger subnet 10.100.0.0/16.

If I start by setting up an access-list to classify this traffic with a high dscp value, am I on the right track here? When setting up an access-list that specifies a dscp value, is that saying to tag the traffic with that vlaue if it matches the access-list, or is it saying to check the traffic to see if it was already tagged with that dscp value?

Thanks in advance for any help.

[Roberto Salazar]

Usually, IP Phones would have DSCP value of 46 already assigned to the voice traffic it generates. Is there anyway you can find this out from the IP Phones documentation or vendor? If that is the case all you have to do is to clasify the traffic with this DSCP value and treat it appropriately. IN switches caes you can trust DSCP on the port. On the router, you'll have to use the Modular QoS configuration like Policy Map with class map.

Please rate all posts.

[mbalik]

I'm trying to find out if there is a default DSCP value. There is a location in the configuration where I can specify the DiffServ / ToS Byte - is there a conversion chart for what DSCP values match up to these values?

[james4627]

Yes, there is a default. I don't know them off hand, but this link may be a start:

http://www.cisco.com/en/US/partner/tech/tk389/tk813/technologies_q_and_a_item09186a00804976d3.shtml

I think the config example listed there IS the default...

Hope this helps.

[mbalik]

Am I correct then that a DiffServ value of 46 is NOT the same as a DSCP value of 46?

[mbalik]

I found one article that notes a DiffServ value of 184 is equal to EF. Everything I'm reading on Cisco hardware works off of DSCP and from what I gather, the DSCP value of 46 is equal to EF. If I say that I work with DSCP value 46 in my Cisco config, will that trust the DiffServ value 184 or do I need to do additional steps in between somewhere?

[mbalik]

Bosalaza,

I have put a DiffServ value of 184 to be tagged by my IP phones. I used a packet sniffer and found that when call traffic is taking place on the Cisco PoE switch, the frames are tagged with EF values.

Could you point me in the right direction for where I need to go next to ensure this traffic always has a certain amount of dedicated bandwidth? For example, on my gigabit uplink, I would want to guarantee the EF tagged traffic always has 5 megabit of bandwidth available for use.

Thanks in advance.

[ariela]

Hi,

normally voice traffic uses the priority queue, for low jittering and delay, and always "normally" we use a 'priority percent 30' to avoid starvation.

Your Cisco PoE switch is a 3550? or what?

Regards

Andrea

[mbalik]

'Priority percent 30' would be fine as well. We have a gigabit backbone and voice traffic is our most mission critical so giving it 30% would not hurt.

Our switches to power the phones (non Cisco phones) are 3560s and the rest of the infostructure is either 3550's, 3560's, or 3750's.

Thanks.

[ariela]

pay attention on 3550:

http://www.cisco.com/en/US/tech/tk389/tk813/technologies_tech_note09186a00801558cb.shtml#topic3

very good example:

http://www.cisco.com/en/US/products/hw/phones/ps379/products_implementation_design_guide_chapter09186a00802a0a05.html

HTH

Andrea