QOS configuration on VOIP only port

[paul amaral]

Sort of an odd question but what is everyone's thoughts on QOS configuration for older routers what don't have hardware QOS and the port will only carry VOIP traffic. Should the QOS configuration still be applied? I asked because I'm not sure what QOS would accomplish on the port/s that only send VOIP packets. 

[balaji.bandi]

what is the model of the router and what IOS version it running

show verion  <--post output.

[Joseph W. Doherty]

If the interface is only caring VoIP traffic, probably often little benefit to enabling QoS, however often isn't always.

You might treat VoIP bearer traffic different from VoIP signally traffic. Also, even if you "tune" just a FIFO queue's buffers, you could also consider that QoS. Lastly, if you have a downstream bottleneck, where you cannot control QoS, you might shape, upstream, at a location where you can provide QoS.

[paul amaral]

Joseph, is it common to even setup QOS on such ports, thats the real question. Often these are 1 gig ports that just carry signal/voip, is it even worth doing any QOS? Also for FIFO queues, do you mean assigning x number of buffers on the HW/software queues? do you have an example. As for shaping I get what you are saying but I'm talking out QOS out to the LAN on a port that will only do VOIP. I never hear what if anything should be done here and how to deal with this situation. I guess I'm trying to figure you what everyone does in this situation if anything at all. 

Paul

[Joseph W. Doherty]

Paul your OP asked about older router ports. In my mind, older router ports, weren't gig, or if they were, almost all the older ISRs didn't support actual gig throughput. Further, if using an ISR, often you had a WAN link of much less bandwidth than supported on the LAN. Lastly, unlike your situation where you're only carrying VoIP traffic, I believe most routers would carry traffic besides VoIP, and in such cases, QoS to ensure a SLA for VoIP becomes much more important.

However, as I noted in my earlier reply, if you're only carrying VoIP traffic, most "traditional" QoS is likely not of much benefit. QoS usually is only of benefit when an interface/link has congestion that is detrimental to the needs of the traffic. It doesn't matter whether the bandwidth is in the K or gig, the impact of congestion, short or long term is what matters.

As to what QoS is common, I believe QoS, generally, is uncommon. More common is the cry, "we need more bandwidth".

In your latest post, you're now asking about QoS on LAN ports. Again, it doesn't matter WAN, MAN or LAN. If there's congestion, to "sensitive" traffic, you're going to have a problem. If not congestion, there won't be a problem.

Even with congestion (which I say is any time there's even one frame/packet waiting), again, it might not be detrimental. If it is, QoS will only help if you can basically trade of one kind of traffic's requirements against another. For example, a bulk FTP file transfer, that fills a link, often will be detrimental to VoIP traffic on the same link. However, if QoS provides priority treatment to the VoIP traffic, the VoIP traffic no longer suffers from the concurrent FTP traffic.

Don't worry much about what othera do, or not, regarding QoS. Determine what your traffic service needs are and whether QoS can help meet them or not.

[paul amaral]

Joseph, the post was, at first for older routers since at the time of the post that is what i was looking at. However to expand on the subject we can use an ISR 1800/1900 etc. So let's take a gig port outward to the LAN that is only doing VOIP/Signal. Do we use QOS? yes/no what benefits do we get. I find it that in a scenario where the WAN is 5 Mbs and the LAN ports are Gig ports segmented by vlans, so data vlan, voip vlan, that I don't use QOS on the VOIP only port. Obviously I use it on the WAN outward etc but on a 100/Gig port with no congestion and doing voip/signal only packets ( 99% at least ) I dont setup any QOS at all. Reading your response that QOS has no real benefit in this case is what I concluded myself but wanted to make sure that is the general consensus, didn't know if i was missing something :) Obviously on logical sub interfaces, WAN links and mix packet VOIP/Data LAN interfaces I setup QOS.

Paul 

[Joseph W. Doherty]

Paul, I think we're on the same page. However, as often the case, anytime you believe "always" applies, there's some exception.

Keep in mind, regardless of the bandwidth, any link can be congested. Further, even when they have overall low utilization, they can still be congested in short term.

For example, consider 1,000 VoIP flows whose packets all hit a gig interface at exactly the same time. The gig interface can only transmit one flow's packet, so 999 VoIP packets will need to be queued. To avoid dropping any, the queue needs to be able to store those packets. So, as I noted earlier, insuring you have sufficient buffers can be considered QoS because by insuring a sufficient number of buffers you're insuring quality of service.

Let's further assume half of those flows are bearer traffic and half signally traffic. Do you need to prioritize bearer over signally?

The answer depends on what happens if one of the received bearer packets is the last to be transmitted. I.e. will it be detrimental to that VoIP conservation?

In the real-world, most of the time, not using QoS, in the situations you believe where you don't need it, will work out just fine. But QoS is can be like insurance. You hope to not need it, but it's great when you do. The decision is yours. You need to evaluate the risk vs. cost of having it or not.