To QoS or not to QoS


Hey all,

Just chatting with a customer about QoS. I have to admit, I’ve never used it.

I was told years ago that QoS was developed back when T1 lines were still 1.5 Mbps symmetrical and that we didn’t need it any longer with the improved speed and gigabit routers and switches. Unless complex, possibly multi site, MPLS things are involved.

So, I’ve never used it. None of the IT companies that have brought me in to manage systems for them have used it.

Are you guys using it? Have I overlooked something important?


Interesting questions. I too have not used it.


Unless the diffserv value eg EF Diffserv 46 is honored across the wan links it will not work , So unless you the ISP and all you edge devices at each hope is configured you have no control over the qos once it leaves your edge device. So practically it is not so easy to get it to work maybe form one office to another that you have a dedicated link or mpls.

The only one you can control are on Vlans with the TOS value the 802.1 q value for you trunk port/tagged port.
With more ISP taking VoIP seriously has Landline fall away it may be better to start marking your packets with 46 for the layer 3 qos and 5 for the local Lan qos.

where it works it works at least you have it set.rather than not setting it at all and a receiving router can use the value but you don’t have it so it ignored.
Well that’s what we doing from this year anyway


Your answer needs some clarification as you have not mentioned the internal network other than to suggest Vlans.

QoS is somewhat fickle, but the reality is that most have an internal network generating more data than what the external network can handle. QoS in a Vlan is layer 2, whereas Diffserv (DSCP) is layer 3. You are correct that QoS is largely ignored once the packets have left the router as it would otherwise require an honor system where all observed some standard to tag the packets accordingly. Otherwise and unfortunately everyone would and will set all their packets to EF (expedited forwarding) thereby making all packets peer to one another once again.

The key however, is to determine the likelihood of your network generating more external traffic than your Internet connection can handle and if so, how you manage the LAN to insure that what needs to be prioritized can be. If you have a highly active GB network that does a lot of cloud work that feeds into a 100Mbs Internat connection with perhaps a VPN or two, then you may indeed need to implement QoS or even internal bandwidth shaping/throttling.

While you can implement QoS at almost all but dumb switch and inexpensive routers, they need to be told what to look for. They do nothing until such time as they see enough data coming in that it starts to reach the threshold of what you have told the devices their available bandwidth is. Only once they sense the threshold being reached do they start their QoS routines and examine the packets and prioritize same. And, while you may have a GB switches and the like, it may not be prudent to tell the switch that its bandwidth availability is 1GB if you have high traffic LANs as this then become a similar scenario to the idea of the Internet supporting QoS and everyone set to EF. This then place all the gate-keeping burden to the router. QoS is not a guarantee, it is a moderator that attempts to match the available resource (WAN) to the demand (LAN) and it will prioritize those packets that have been prioritized as much as it can, but it must still attempts to serve the other packets. As the congestion grows, the router will start to discard lower rated packets and if it continues, the higher rated ones will also be dropped. Simply put, it becomes overwhelmed and is no longer able to bring order to chaos, but it will keep trying.

I have a couple of sites using WISP with fairly low bandwidths as they can get Internet any other way and a couple can’t get phone service other than VoIP and cellular was too expensive. I have to use it so the voice is protected.


Is there a downside to always setting it up for VoIP?

Do you setup both QoS in layer 2 and Diffserv (DSCP) is layer 3 at the same time, or do you just use one? If only one, which is preferred?


I only use DIffserv for the most part. If using a VLAN, then there as well.

While I do run across Fortinet, Sonicwall, UBNT and the occasional other, I tend to try and use Draytek which has the function built-in as it sniffs for the SIP port and then reads the associated messaging to get the RTP ports and then prioritizes it to the Internet, but again only when congestion occurs. It will still show you have the traffic is classified however.

As the phones communicate with the UCM and not to the Internet, given the post topic, then the UCM has the provisions for QoS in ToS for getting to the router.

As I have more than one VoIP capable device behind the Draytek and can only specifically identify one VoIP port, the other devices I have categorized into Class 1.


@lpneblett, what settings do you use for QoS? Scanning the Internet I found this; do you agree?


CS3 for SIP
EF for RTP
A41 for Video.

No, to your chart with regard to SIP.

SIP is a query response protocol whereas RTP is not. There is no point in putting SIP in the same class when, if a response is not seen to the query, a resend will occur in 500ms, 1sec and so on until t1 timer expiration. The RTP will not start without SIP nor move without SIP telling it what to do. Class3 is for critical applications.


Thank you for you reply. And sorry to be so dense on this topic; maybe not enough coffee yet?

What values do you suggest for 802.1p/CoS Values for SIP and RTP?

Do you specify both? Why or why not?

I am really trying to understand this, so I appreciate you patience and understanding with me.


Attempting to do Qos on commercial link is near impossible you can set your edge router like a mikrotik and reserve your bandwidth but that its.
Once your packets leave your router you got no control… If you used the EF value of 46 it will have that tag and you can use the diff-serve value to isolate that traffic and apply it to a queue on your router before traffic exits has it is natted ( hope I make sense here)…

*** Important for voice provider when using valns where network is set up by 3rd party ****
" This is my personal view has it can’t do no harm and if used can benefit in times of congestion"

If we focus on the Lan set up where Vlans are in use ( 802.1q = valns) we have 5 vlans ( lets assume you do not manage the network it is set up by the an outside IT company and they gave you VLAN 30 for voice)

802.1q is the Vlan tag where we have

vlan10 data -------------------------|
vlan20 security cameras---------|
vlan30 voice ------------------------| >> All these vans transverse across a single port on the switch >>>>>>>>
vlan40 manaement ---------------|
vlan50 xxxxxxxx -------------------|

continued over a single cable >>>>>> now we have 5 data from 5 vlans how how does the switch know what to fwd it will just fwd has it can lets call it best effort but most switches when trunking (set up for Vlan) are aware of a “tos” or “cos” value voice is set to 5 for highest priority is 7. if you set it to 5 your swith that is set up to trunk the Vlans will honor your value and give you voice valn30 priory … well assuming that your It company did not set 5 for everything else 6 and 7 is for network control and signaling handshake etc. so its not used or should not be used.

so when you set you Vlan value which you with have to set for your voice to work remember the 3rd party IT company gave you Vlan 30 you with also set the tos or cos value ( same thing different name) to 5 so when there is congestion at the switch port your voice will get priory .

yes 802.q is the valn tag and 802.1p is it priority in essence how your QOS will be applied
802.1q = valn30
tos/cos layer2 qos = 5
you not setting it for sip or RTP all packets that leave the device will be marked .


3 for SIP and 5 for RTP.


Great conversation! Thank you all for your feedback.

@Telecomsolutions, I totally forgot about bandwidth reservation. I’ve got a couple Mikrotiks setup and use bandwidth reservation to ensure all other traffic will not overwhelm the VoIP traffic (assuming no issue with the ISP). While I’m not able to change the setup, as it’s fixed a number of issues for these customers, the complaint I have is that I need to have a total less than the maximum bandwidth the customer is paying for.

For example. Say the customer has a 100 Mbps connection but, with testing, I find they sometimes get as low as 92 Mbps. Bandwidth limiting only works if the VoIP limit, and the Data limit do not exceed the lowest, tested bandwidth the customer is actually getting. So, I setup the VoIP for x Mbps, and the Data for y Mbps where X+Y+ 92 Mbps or, at least some number I’m comfortable with.

Bandwidth limiting is a blunt force tool. You can end up leaving large chunks of potential bandwidth on the table. But, for my customers who’ve had troubles. It freaking works amazing.


My interpretation of this conversation is, QoS is not important unless you’re fixing a problem.

Given all the VoIP providers in the world (Ring Central, 8x8 and so on) that do not require or even ask for QoS to be setup on a network.



It is not the provider’s responsibility to inform or suggest QoS about your network. However, many have some form of testing to indicate whether or not you can support VoIP. So bandwidth is a concern and while you may be able to support VoIP, that single snapshot test does not indicate your utilization over what might be a busy network at certain times.

To say you don’t need QoS, is a statement that only holds true until you do and you typically only find that out when the client complains. And, I don’t know that I term QoS as fixing a problem. The key is that you don’t want QoS to engage except as the occasional fail-safe mechanism. If it engages frequently, then you have a bandwidth issue, not a QoS issue. It’s like a band-aid in that it stems the flow of blood, but I sure don’t want to wear it forever. At some point, I want the wound to heal so it won’t bleed.


I setup QoS by default - using Draytek routers on the most part. But I would expect that generally it is not being used, given the bandwidth availability. However, it is there in case there are peaks in usage that could impact voice. Everything is online now, so there is more and more potential for data traffic to impact voice. But, as has already been stated, if QoS is being over used by the router then you have a bandwidth problem not a QoS one…


bandwidth has nothing to do with it, this is a mistake many people make,
you can also have 1 Tera of bandwidth, but if you do not set the router / firewall correctly, if for example a user downloads a movie, he is granted all the bandwidth possible, so in that period of download the bandwidth passes from 1 Tera to zero (theoretically obviously).

Yesterday I was with a big customer who thought exactly this
“I have 100/100 symmetric so THERE ARE NO PROBLEMS”,

I showed him that simply using ZOOM he had irregular voice problems (he also had a “toy” firewall), but this is an additional “serious problem”


Are you suggesting that the Draytek routers are a ‘toy’ firewall?

I’d argue bandwidth has quite a lot to do with it…it is after all the whole reason you need QoS in the first place. :slight_smile: If you don’t have enough then QoS needs to do some work. If you have enough available then QoS does nothing (unless you get a bit of a peak).

In most of our setups …
“But I would expect that generally it is not being used, given the bandwidth availability. However, it is there in case there are peaks in usage that could impact voice”

Curious what bandwidth is being used by ZOOM on your 100/100 network that is impacts voice. Or could it be the ‘toy’ firewall is not coping with the throughput?


Damiano, Larry, James, David, Neeraj, I am always applying QOS on rtp at the firewall and internally on the network to attempt to reduce the local issues that can cause packet loss, prioritisation issues etc…

I always use Mikrotik with Queues to ensure that first of all the RTP packets are honoured as the highest priority over anything else. Video can be prioritised but a lot of IT people forget that RTP is real time, not like data packets - video included to be best effort…

RTP in the networks I look after, always take the forefront - can consume all bandwidth as a Priority 1 everything else can compete with the remainder bandwidth.

As Larry has pointed out that if there is no QOS on the link to the provider that the QOS on the inside of the network is not honoured outside of the network, so you might still come across packet loss or other congestion that can cause voice problems.



I have been using Draytek for 15 years, where did you read that I wrote that they are toys?


We use QoS on the switches and routers. Unless you have a private WAN link it’s likely the wild west for your packets once they leave your router.