QoS Configuration Guide: Prioritizing Network Traffic Effectively
When bandwidth is limited, Quality of Service ensures critical applications get the resources they need. Learn how to implement QoS policies that work.
What Is QoS?
Quality of Service (QoS) is a set of technologies that manage network traffic by prioritizing certain types of data over others. Without QoS, all traffic competes equally for bandwidth. With QoS, you can ensure voice calls stay clear even when someone is downloading large files.
The Core Problem
Networks are shared resources. When demand exceeds capacity, something has to give. QoS lets you decide what gets priority rather than leaving it to chance.
QoS Mechanisms
Three primary techniques control traffic behavior:
Classification & Marking
Identify traffic types and tag them with priority values. Uses DSCP (Differentiated Services Code Point) in the IP header or 802.1p CoS tags at Layer 2.
Queuing
Place packets into different queues based on their markings. High-priority queues get serviced first. Common methods: Priority Queuing, Weighted Fair Queuing (WFQ), Class-Based WFQ.
Traffic Shaping & Policing
Control the rate of traffic. Shaping buffers excess traffic and smooths bursts. Policing drops or re-marks traffic that exceeds limits.
DSCP Values and Traffic Classes
DSCP uses 6 bits in the IP header to mark priority. Common values:
| Class | DSCP Value | Typical Use |
|---|---|---|
| EF (Expedited Forwarding) | 46 | Voice, real-time video |
| AF41 | 34 | Interactive video |
| AF31 | 26 | Signaling, call setup |
| AF21 | 18 | Business critical apps |
| CS1 | 8 | Scavenger/bulk data |
| Default (BE) | 0 | Best effort traffic |
Important: DSCP markings must be consistent end-to-end. If different devices interpret markings differently, QoS breaks down. Document your policy and ensure all devices follow it.
Designing a QoS Policy
Follow these steps to create an effective QoS configuration:
- 1.Inventory your applications: List all traffic types on your network and their requirements (latency, bandwidth, jitter tolerance).
- 2.Define traffic classes: Group applications with similar requirements. Keep it simple - 4-8 classes is typical.
- 3.Assign bandwidth allocations: Decide what percentage of bandwidth each class gets. Voice typically needs only 10-15% but requires priority.
- 4.Configure marking: Determine where traffic gets classified and marked (usually at network edge).
- 5.Deploy and test: Implement policies in stages. Monitor to ensure traffic is being classified correctly.
Example QoS Policy
A typical enterprise QoS configuration:
| Class | Bandwidth | Traffic Types | Treatment |
|---|---|---|---|
| Voice | 10% | VoIP, Teams audio | Priority queue (EF) |
| Video | 20% | Video conferencing | Low-latency queue (AF41) |
| Business Critical | 25% | ERP, CRM, database | Guaranteed bandwidth (AF31) |
| Default | 35% | Web, email, general | Fair queuing (BE) |
| Scavenger | 10% | Backups, updates | Best effort, deprioritized (CS1) |
QoS for WAN Links
QoS is most critical on bandwidth-constrained WAN links:
MPLS Networks
Providers typically offer 3-6 traffic classes. Coordinate your DSCP markings with their class definitions. Markings may be rewritten at the provider edge.
Internet VPNs
DSCP markings are often stripped on the public internet. QoS only works on segments you control. Use traffic shaping at your edge to prevent congestion.
SD-WAN
Modern SD-WAN solutions include application-aware QoS. They can identify traffic by deep packet inspection and apply policy automatically across multiple paths.
Monitoring QoS Effectiveness
QoS requires ongoing monitoring to verify it's working:
- -Queue statistics: Monitor drops and tail drops per queue. High drops in priority queues indicate misconfiguration.
- -Class counters: Verify traffic is being classified into expected queues. Unmarked traffic defaults to best-effort.
- -Application metrics: Track latency and jitter for real-time applications. If voice quality is poor despite QoS, something is misconfigured.
# SNMP OIDs for queue monitoring (Cisco example)
cbQosQueueingStats: 1.3.6.1.4.1.9.9.166.1.18
- cbQosQueueingCurrentQDepth
- cbQosQueueingMaxQDepth
- cbQosQueueingDiscardByteCountCommon QoS Mistakes
Too Many High-Priority Classes
If 80% of traffic is "priority," nothing is actually prioritized. Be selective. Voice and real-time video deserve EF; most applications don't.
Trusting Endpoint Markings
Applications can mark their own traffic as high-priority. Always re-classify at the network edge based on your policy, not what endpoints claim.
Forgetting Return Traffic
QoS on your outbound link doesn't help if your ISP's return traffic is congested. Consider both directions in your design.