General Question on QoS

[Deepak Kumar]

Hi everyone, 

The QoS is my week point and I want to clarify ASP. Here are some stupid questions that I want to answer from you as you are expert of Cisco and QOS magic. 

1. Is it compulsory to start marking voice traffic from IP Phone?

2. If IP phone is not marked traffic than how to access switch identify the traffic for marking?

3. If I am not configured QoS on Access switch but it is applied to Core switch and WAN router. Is it beneficial for me?

Sorry in advance for these stupid questions.

Regards,

Deepak Kumar

[Peter Paluch]

Hi Deepak,

First things first :) Please do not apologize for asking, and do not call your questions stupid! These forums are here to answer exactly these types of questions.

I will try to answer your questions to the best of my knowledge, but I sincerely hope that Joe Doherty also joins this thread and shares his view since I consider Joe to be an authority on this topic - sincerely.

1. Is it compulsory to start marking voice traffic from IP Phone?

This depends on what your goals and needs are. Without marking certain traffic differently, it will not be treated differently in the network - it as simple as that. If your network is overprovisioned with bandwidth, and hardly has any congestion, packet drops, or excessive buffering, marking the voice traffic is not going to make much difference, because the queuing mechanisms that give preferential treatment to certain traffic classes won't kick into action anyway. But if you do have choke points in network, places where traffic starts to be queued, potentially even being dropped due to congestion, then marking the voice traffic is absolutely required to make it stand out.

Of course, I did not emphasize it before since it seems to be so natural, but it is likely good to call it out: Your network must be configured to recognize different markings and to give them appropriate treatment. Marking traffic by different DSCP values is not going to have any effect if the network itself ignores these markings.

2. If IP phone is not marked traffic than how to access switch identify the traffic for marking?

There are not many options - switches typically have limited features to identify types of traffic.

One of the simplest approaches would be to be define a two-stage policy using ACLs:

• All traffic coming from the phone's IP address and destined to a known TCP/UDP port for signalling (SIP, SIPS, SCTP) would be recognized as signalling and assigned the DSCP value of AF41, for example.
• All other traffic coming from the phone's IP address would be treated as media and assigned the DSCP value of EF.

3. If I am not configured QoS on Access switch but it is applied to Core switch and WAN router. Is it beneficial for me?

This very much depends on where the congestion occurs. If there is no congestion on the access layer then the QoS mechanisms would likely not do anything, anyway.

There is a simple rule to remember, though: The moment you have a congestion even on a single network device, and that device does not honor QoS markings, you have essentially nullified your whole QoS policy, since that device will treat all traffic in the same way and not treat the important flows preferentially exactly when they needed it the most.

Feel welcome to ask further!

Best regards,
Peter

[Dennis Mink]

its not compulsory to mark at the phone at all. its just much simpler to do it this way in a cisco Uc deployment, where your cucm basically tells the phone which dscp values to use for video.voice and signalling. if you just tell your switch to trust these values than you do  need to apply all sorts of acl on your router/switch to define this sort of traffic.

[Joseph W. Doherty]

#1 No, it's not, as noted by Dennis.

#2 It can be identified in various other ways. As also noted by both Peter and Dennis.

#3 As Peter describes, it really depends whether there's congestion, more specifically, whether there's "enough" congestion that it's adverse to the service needs of the applications and also whether QoS can provide the service levels required. As you also specifically asked about a WAN router, the foregoing still holds, but it's often one of the most common places you'll see congestion going from a LAN to a WAN as often the latter supports less bandwidth.

BTW, for number 1, marking traffic is really for efficiency. It allow devices to quickly analyze a packet's service requirements by examination of a single byte. Again, you don't need to mark packets to analyze them and further just because a packet is marked doesn't always mean it will be subject to analysis and given any different service treatment. (Also understand, if packets are marked, they might also be remarked along the way.)

Also BTW, when traffic ingresses a network QoS trust boundary already marked, it's often a good idea to somehow verify the marking is valid for the traffic.