QoS principles (small network with DATA, Voice and Video)

[vladyslav_s]

Hi guys,

Could you clarify for me when QoS configuration will kick in, after the whole link is utilized or all the time?

I have a small office with 15-20 users, 5 servers, a couple of video endpoints and VoIP PBX (Avaya IPO500) for phone calls.

Everything is connected via 2960X switches and ASA5520 is on the edge of the network. Internet connection is 100/100Mbps.

VoIP is currently not separated from Data but there were no issues so far.. To separate them is on my list...

I'm thinking of adding video MCUs (30 ports, so 30*1Mbps calls) to the network (in DMZ) and want to be sure the video (UDP) packets will be prioritized over Data. MCUs have Diffserv configuration.

I had an impression that I need to configure prioritization for that but after talking with some people I was told that prioritization will work only after my Internet link (100/100) is fully utilized...

I was thinking that in the best scenario the video packets need to leave outside interface first but looks like it doesnt matter until the whole bandwidth is not used...

Thanks!

[Mark Malone]

Hi so qos wont work over an internet link unless its tunnelled end - end , is it setup like that ?

if its is make sure you apply it to the tunnel as well

[vladyslav_s]

Let's say, packets from my internal network arrive to ASA's outside interface in the following order (Data, Video) ->DDDVVDVDVDDV->Internet

Can I prioritise Video (so it'll leave first) and change it to DDDDDDVVVV->Internet

Also, can I prioritise incoming traffic from the Internet on my ASA? Based on destination IP or ports?

Thanks.

[Mark Malone]

You should be able to set it to leave first alright that shouldn't be an issue one way by setting af41 in a priority queue so its serviced first before anything but nothing upstream will honour the markings as you have no control over it.

Also, can I prioritise incoming traffic from the Internet on my ASA? Based on destination IP or ports? No this wont work

[Joseph W. Doherty]

Hi so qos wont work over an internet link unless its tunnelled end - end

I would say, a tunnel across the Internet doesn't guarantee QoS will be useful and not having a tunnel across the Internet also doesn't mean QoS isn't useful.

The "key" to generally having useful QoS across the Internet is if you "control" the bandwidth at your Internet ingress/egress points, and that the Internet doesn't congest between your Internet ingress/egress points.

For example, consider two sites, each with a 100 Mbps Ethernet Internet connection.  Assuming that both those sites only have inter-site traffic, you effectively have a 100 Mbps p2p connection.  You can add physical egress QoS, to manage congestion just as you would any other p2p 100 Mbps link.

Or for example, consider one site has a 100 Mbps Internet connection and two other sites have a 50 Mbps Internet connections.  Using a logical hub-and-spoke topology, and shaping 50 Mbps at the 100 Mbps (hub) site, traffic to each 50 Mbps (spoke) site, you effectively have two 50 Mbps p2p connections.  At the 50 Mbps sites, you can add physical egress QoS, to manage congestion just as you would any other p2p 50 Mbps link.  At the 100 Mbps site, you can use QoS to manage the congestion on the two 50 Mbps shapers, much as you would if you were using QoS on two 50 Mbps physical egress interfaces.

Where you'll have problems, with effective QoS, is if you try to "share" Internet bandwidth with general Internet traffic, or in my example of 3 sites, if you try allow any-to-any traffic flows.

[Mark Malone]

Thanks joseph that's a good example , the one issue im always concerned about qos when not tunnelled on the internet is the ISP wont provide any priority to the marked TOS packets , if its not tunnelled will it not defeat the purpose of Qos if everything is given the same priority anyway by the ISP when it reaches there network , at least when your in the tunnel you can make sure the packets are not re-written  end - end

[Joseph W. Doherty]

When it comes to ToS markings, notice I didn't mention them.  One reason I didn't, ISPs generally don't provide any different service treatment based on ToS markings.  For what I described, ToS markings, unless the ISP is treating them differently, doesn't make any difference.

You're correct, if you desire to "protect" your ToS markings from ISP changes, encapsulating the packet should generally guarantee that. However, ISPs generally also don't remark your packets, as again, they tend to just ignore your ToS markings.

If your ToS markings are needed, between sites, besides possibly protecting them in a encapsulated packet, you could also remark them, as needed, upon ingress from the Internet.

[Joseph W. Doherty]

Could you clarify for me when QoS configuration will kick in, after the whole link is utilized or all the time?

Depends on the QoS configuration.

As you also mention "whole link is utilized", you might be thinking that QoS is only really needed when that happens.  QoS might or might not be needed in that situation, but also whole link utilization is often every time you send a packet.

Whenever there's congestion that's adverse to your traffic's service needs, QoS might be a method to manage that congestion so your traffic's service needs are met.

I had an impression that I need to configure prioritization for that but after talking with some people I was told that prioritization will work only after my Internet link (100/100) is fully utilized...

I was thinking that in the best scenario the video packets need to leave outside interface first but looks like it doesnt matter until the whole bandwidth is not used...

"Some people", unfortunately, are often mistaken in their understanding of QoS.  Again, a link can be fully utilized during transmission of any single packet.

"doesnt matter until the whole bandwidth is not used." Well, that might be true, but it depends on how you understand "whole bandwidth" being used.