---
title: "How we test QoS: classes under 3:1 congestion | TEST4NET"
description: "QoS on the CloudLAB bench: DSCP and 802.1p marking end to end, queues per class, policers and shapers, latency and jitter per class at 3:1 congestion"
image: https://test4net.com/hubfs/blog-covers/how-we-test-qos.png
---

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# How we test QoS: DSCP, queues and voice on a full link

![](https://test4net.com/hubfs/IMG_0070.jpg) [Alexander Zemskov](https://test4net.com/blog/author/az) October 29, 2026 4 min read

[English](https://test4net.com/blog/how-we-test-qos) [Русский](https://test4net.com/ru/blog/how-we-test-qos) [Հայերեն](https://test4net.com/hy/blog/how-we-test-qos)

![How we test QoS: DSCP, queues and voice on a full link](https://test4net.com/hs-fs/hubfs/blog-covers/how-we-test-qos.png?width=1600&height=900&name=how-we-test-qos.png)

In this article

A router datasheet lists 8 queues per port, strict priority, WRED and hierarchical shaping. The design document marks voice as EF and puts it in the priority queue. Both are true, and the call still breaks up at 9 a.m. On an idle link QoS does nothing. It starts to work the moment an egress port receives more than it can send, so that is the only state in which we test it.

## The bench

The device under test is a [router or a switch](https://test4net.com/solutions/routing-switching?hsLang=en), or an [SD-WAN edge](https://test4net.com/solutions/sd-wan?hsLang=en), with 1G, 10G or 100G ports (400G and faster per project) cabled to the traffic generator. The generator sends one stream per class: voice, video, business applications, bulk transfer. Each stream carries its own DSCP or 802.1p marking, a sequence number and a timestamp in every frame. Three input ports feed one egress port, which gives 3:1 oversubscription (an example; we set the ratio your uplinks will see). On an SD-WAN edge the WAN side goes through a WAN emulator with the contracted rate, delay and jitter of each underlay.

The policy is yours: class maps, trust boundaries, queue weights, policers and shapers from the configuration export. Bulk traffic is real TCP that takes whatever the scheduler leaves, so the voice queue competes with the traffic it will meet in production.

## What we check

- Marking: DSCP and 802.1p values at the egress for every class, on trusted and untrusted ports, and whether the mark survives a tunnel, a VLAN rewrite or an IPsec header where the design needs it
- Classification: each class lands in its own queue; per-queue counters on the device are read next to the per-class counts of the generator
- Scheduling at 3:1: the delivered rate of each class against its share in the policy, and the cap on the priority queue so it cannot starve the others
- Latency and jitter per class: percentiles per stream, voice and video against the budget in your design while bulk traffic fills the link
- Policing: offered load 25% above the limit; the received rate must fall back to the limit with no loss among conforming frames
- Shaping: the same excess held in the buffer, the delay it adds, and the burst size the shaper absorbs before it drops
- H-QoS where the design has it: a parent shaper per site or per subscriber with class queues inside, and whether one site's bulk traffic reaches another site's voice

Each item has a pass criterion written before the run, such as a maximum jitter for voice or zero loss for conforming frames.

## What usually breaks

The mark. Each feature works alone: VXLAN, an egress ACL, DSCP remarking. Put all three on one port and the DSCP value is gone on the way out, so every hop after it treats voice as best effort. A test of one feature at a time misses it, which is why our [functional checklist](https://test4net.com/services/functional-conformance-testing?hsLang=en) runs the features together.

Second: the shaper. A branch edge has a 1G port and a 200 Mbps contract (an example). Without a shaper at 200 Mbps the device never sees congestion, its queues stay empty and the provider's policer drops whatever it likes, voice included. With the shaper the queueing moves into the box you configured.

Third: classes that pass one by one and fail together. A typical [Y.1564](https://test4net.com/methodologies/y-1564?hsLang=en) activation: three CIRs add up to 620 Mbps, and the policer on the aggregation node is set to 450 Mbps, a figure from an earlier version of the order. Alone, no service reaches it. Together they do, and the data class starts losing frames.

## What is in the report

A table per class: offered and delivered rate, loss, latency and jitter per percentile, at each load step and at 3:1. The marking table: value in and value out, per port and per hop. Policer and shaper accuracy against the configured rate. Per-queue counters from the device beside the generator counters. The policy export, the firmware and the port plan, so the vendor can reproduce any item.

## Where to start

Send us the device model, the firmware, the QoS policy export and the traffic mix per class. We answer with a draft test plan: the items, the bench layout and the dates. The report is yours to keep and to show to the vendor. [Write to us](https://test4net.com/contacts?hsLang=en) or look at [what else the bench does](https://test4net.com/services?hsLang=en).

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Written by

### [Alexander Zemskov](https://test4net.com/blog/author/az)

Alexander Zemskov leads network and security testing at TEST4NET, an independent, vendor-neutral testing lab in Yerevan, Armenia. With 19+ years in performance and load testing on IXIA, Spirent, Keysight, Xena and Xinertel platforms, he runs NGFW, WAF, IPS, DLP and SASE evaluations for telecom operators, banks and system integrators.

[LinkedIn](https://www.linkedin.com/in/azemskov/) [Website](https://test4net.com/?hsLang=en)

[Previous article How we test VNFs: vCPUs, SR-IOV and a noisy neighbour](https://test4net.com/blog/how-we-test-virtual-network-functions?hsLang=en) [Next article How we test TCP throughput: window, RTT and the SLA dispute](https://test4net.com/blog/how-we-test-tcp-throughput?hsLang=en)

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  },
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    "logo" : {
      "@type" : "ImageObject",
      "url" : "https://test4net.com/hubfs/test4net-logo-fixed.svg"
    },
    "name" : "TEST4NET LLC"
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}
```

```json
{
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  "@id" : "https://test4net.com/#organization",
  "@type" : "Organization",
  "address" : {
    "@type" : "PostalAddress",
    "addressCountry" : "AM",
    "addressLocality" : "Yerevan",
    "postalCode" : "0033",
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  "image" : "https://test4net.com/hubfs/og-t4n.png",
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  "telephone" : "+374-60-88-88-99",
  "url" : "https://test4net.com/"
}
```

```json
{
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  "@id" : "https://test4net.com/#website",
  "@type" : "WebSite",
  "inLanguage" : "en",
  "name" : "TEST4NET",
  "potentialAction" : {
    "@type" : "SearchAction",
    "query-input" : "required name=search_term_string",
    "target" : {
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  },
  "publisher" : {
    "@id" : "https://test4net.com/#organization"
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}
```

```json
{
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  "@type" : "BreadcrumbList",
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    "item" : "https://test4net.com/blog",
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    "name" : "How we test QoS: DSCP, queues and voice on a full link",
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}
```