Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

QoS newbie questions

Hi All,

I am finally getting around to dealing with the voip issues in my inherited network. It seems voip was put into place with no thought at all. the siemens ip phones are on the same vlan as data and there is absolutely no qos involved. I have moved the ip phones to their own voice vlan, but as far as qos goes i am a total newbie. I have no clue where to start. Should I be looking to do this at L2 or L3? I have read in cisco docs that switches do not tax the main cpu when doing cos like routers do. I have all cisco switches and most if not all are L2 and L3. Is it possible to let a specific vlan (my voice vlan) have priority over everything else? is this the best way? Can someone give me direction?

TIA,

R

9 REPLIES

Re: QoS newbie questions

Auto QOS is a quick fix for switches but personally I prefer to learn the commands and truly understand whats going on. Router queues usually require more time and effort to configure correctly. Ultimately the best thing for you to do is to take a day and read through the QOS SRND. It's the best overall all in one resource for QoS. Depending on your hardware though you still might need to look up configuration for your gear but it has most of the common used equipments configurations in there. I was a QoS newbie when I first read it and I feel real confident now on my ability to deploy QoS.

Here is the link

http://www.cisco.com/application/pdf/en/us/guest/netsol/ns432/c649/ccmigration_09186a008049b062.pdf

Please rate any helpful posts

Thanks

Fred

New Member

Re: QoS newbie questions

Thanks for the info! I was actually right at the beginning of reading this document before you sent it and agree that it is proving to be useful.

One thing I am concerned about, is how I can use these models explained by the paper when I do not have cisco ip phones (which means no cdp to the ip phones). It is hard for me to look at the models described and pick the appropriate one. It seems I will be in a "Untrusted IP Phone + PC".

New Member

Re: QoS newbie questions

The doc provided is a great resource. I have all our switches setup with QoS but through experience have seen more issues as a result of L3 than L2. The auto qos on the switches has worked great for me.

The documents are a bit intimidating because of all the different classes they show. In my network, I have a very simple QoS config based on DSCP values. I use Cisco phones and they set the DSCP correctly for the RTP and control traffic. But, the cool thing is that in IOS is you can classify the traffic based on a variety of fields. Do your phones mark the traffic (IP precedence or DSCP?), if not you can use access-lists to classify traffic based on protocol, source/destination, etc.

I use 4 simple classes - Voice, Control, Video and default. I put voice and video into the same priority queue(there is only one priority queue, some argue to separate voice and video).

Here is some of my config to help get you started:

class-map match-any CONTROL

match ip dscp cs6

match ip dscp cs3

match ip dscp af31

class-map match-any VIDEO

match ip precedence 5

match ip dscp af41

class-map match-all VOICE

match ip dscp ef

policy-map V3PN

class VOICE

priority percent 33

class CONTROL

bandwidth percent 10

class VIDEO

priority 640

class class-default

fair-queue

Just apply this to an interface using service-policy output V3PN and you're done!

Good luck.

New Member

Re: QoS newbie questions

Thanks so much for the config. I am finding that the document is very good info, but like you said is a bit intimidating with so much info being given.

I took a look at the config on my ip phones and they do indeed mark their traffic on L2 and L3, so I suppose this is a good thing! I will look at your config more closely and see how I can possibly impelent it.

One question I do have is the following. Wher you have "match ip dscp af41" can I use the numbers associated with DSCP instead of the letter values or does it matter? My phones relay the values back as numbers (EF = 46). or if I leave this the way it is, will cisco know what to do with the 46?

Thanks!

Re: QoS newbie questions

I have used both the DSCP values and PHB values in my configurations. I mainly work with 6500 CMM and 3700/3800 series routers. The raw command reference shows it with the numerical DSCP value but again I have used both. Most platforms should allow both but always double check it in the command reference for the IOS/Platform.

Thanks

Fred

New Member

Re: QoS newbie questions

You won't have any trouble. There are 3 keywords you can use with classifying: BE, AF and EF. BE=best effort, AF=assured forwarding, EF=Expedited forwarding.

BE=DSCP 0

EF=DSCP 46

AF has more values and is based on classes. But, ultimately, everything gets converted to the binary equivalent.

Good luck.

New Member

Re: QoS newbie questions

Thanks! I think I need a little more help on the interfaces. I do not quite understand where you put the service policies. how do you know which interfaces and which way (out or in)?

New Member

Re: QoS newbie questions

keeping with the example

class-map match-any CONTROL

match ip dscp cs3

match ip dscp af26

match ip dscp af31

class-map match-all VOICE

match ip dscp ef

policy-map V3PN

class VOICE

priority percent 33

class CONTROL

bandwidth percent 10

class class-default

fair-queue

over a wan

int s0/0/0

service-policy output V3PN

New Member

Re: QoS newbie questions

control traffic will not be 10% - it will include RTCP and whatever signalling protocol your phones use (SIP,etc). 5% is the standard, but that is a little high as well.

The 33% for the priority queue is a conservative number that is used to ensure that if you do not optimize your config it will still work. You do not want to dedicate so much to the priority queue that it becomes FIFO, but you can identify the one-way delay between devices and working within the jitter buffer size increase the percentage to a point that does not make the queue FIFO, and it will increase your concurrent call capacity.

RTP packet size with headers/port speed = serialization delay

check the tx-queue size, if it is set to 2 set it to 1, this will reduce the inter-packet delay for RTP packets and reduce your one-way delay = more bandwidth for priority queue.

224
Views
8
Helpful
9
Replies