1.5 Interpret wired and wireless QoS configurations
📘CCNP Encore (350-401-ENCORE-v1.1)
What Is a QoS Policy?
A QoS policy is a set of rules that tells a network device how to treat different types of traffic.
A QoS policy answers questions like:
- Which traffic is important?
- How much bandwidth should it get?
- What happens when the network is congested?
- Should some traffic be delayed or dropped?
In Cisco networks, QoS policies are created using a policy-map and are applied to interfaces, WLANs, or wireless profiles.
Why QoS Policies Are Needed
Networks often carry multiple types of traffic at the same time, such as:
- Voice traffic
- Video traffic
- Application data
- Management traffic
A QoS policy ensures:
- Important traffic gets priority
- Delay-sensitive traffic is protected
- Network congestion is controlled
- Bandwidth is fairly shared
Without QoS policies, all traffic is treated the same, which can cause poor performance during congestion.
Where QoS Policies Are Applied
QoS policies can be applied at different points in the network:
1. Wired Network
- Switch interfaces
- Router interfaces
- Uplink and downlink ports
2. Wireless Network
- WLANs (SSIDs)
- Wireless profiles
- Per-user or per-application policies
The concept is the same for wired and wireless QoS—the policy controls traffic behavior.
Components of a QoS Policy
A QoS policy is built using three main components:
1. Class Map
2. Policy Map
3. Service Policy
1. Class Map (Traffic Classification)
A class map is used to identify and group traffic.
It answers:
“Which traffic am I talking about?”
Traffic can be classified based on:
- DSCP values
- IP precedence
- Protocol
- Access control lists (ACLs)
- Application type
Purpose of Class Map
- Separates traffic into different classes
- Each class will receive a specific QoS treatment
Exam Note
Classification always happens before any QoS action is applied.
2. Policy Map (Define Traffic Behavior)
A policy map defines what action to take for each traffic class.
It answers:
“What should I do with this traffic?”
A policy map can:
- Assign priority
- Guarantee bandwidth
- Limit bandwidth
- Mark packets
- Queue traffic
- Drop traffic if needed
Example Actions (IT Context)
- Give priority to voice traffic
- Reserve bandwidth for video traffic
- Limit bandwidth for background applications
- Mark packets so other devices recognize priority
Common QoS Actions in a Policy Map
1. Priority Queuing (LLQ – Low Latency Queue)
- Used for delay-sensitive traffic
- Traffic is sent first before other queues
- Must be rate-limited to avoid starving other traffic
Exam Tip:
Priority queues are strict and can starve other queues if not limited.
2. Bandwidth Guarantees
- Reserves a minimum amount of bandwidth for a class
- Traffic gets at least the configured bandwidth during congestion
3. Traffic Shaping
- Smooths traffic by delaying packets
- Prevents sudden bursts
- Used mainly on outbound traffic
Key Point:
Shaping delays traffic, it does NOT drop packets immediately.
4. Traffic Policing
- Enforces a strict traffic rate
- Excess traffic is dropped or marked down
Difference from Shaping:
- Policing drops traffic
- Shaping buffers traffic
5. Packet Marking
- Changes packet markings like DSCP
- Helps downstream devices recognize priority
Why it matters:
- QoS works best when markings are consistent across the network
3. Service Policy (Applying the Policy)
A service policy is how a QoS policy is attached to an interface or wireless profile.
It answers:
“Where should this policy be applied?”
Direction Matters
- Input: Traffic entering the interface
- Output: Traffic leaving the interface
Most congestion happens on output, so QoS policies are commonly applied in the output direction.
QoS Policy Workflow (Very Important for Exam)
The correct order of QoS operations is:
- Classification – Identify traffic
- Marking – Label traffic (optional)
- Queuing – Place traffic into queues
- Scheduling – Decide which queue sends packets first
- Congestion Management – Shape or drop traffic
Exam Question Alert:
Cisco often tests your understanding of this order.
QoS Policies in Wired Networks
In wired networks:
- Policies are applied on switch or router interfaces
- Used to manage uplinks and downlinks
- Works with DSCP and CoS markings
Trust Boundary
- Devices decide whether to trust incoming markings
- If not trusted, the device re-marks traffic
Key Concept:
QoS policies are effective only if markings are trusted and consistent.
QoS Policies in Wireless Networks
Wireless QoS policies work similarly but are applied at:
- WLAN level
- Wireless profiles
- Per-user or per-application basis
Wireless QoS Characteristics
- Maps traffic to wireless access categories
- Handles shared medium limitations
- Prioritizes delay-sensitive traffic
Important Exam Concept:
Wireless QoS still uses classification, marking, and queuing—only the application point changes.
Default Class in QoS Policy
Every policy map has a default class:
- Matches traffic not matched by any class map
- If not configured, traffic is handled with best effort
Exam Tip:
If traffic does not match any class, it always goes to the default class.
Common QoS Policy Mistakes (Exam Awareness)
- Forgetting to apply the policy using service-policy
- Applying policy in the wrong direction
- Not limiting priority queues
- Mixing policing and shaping incorrectly
- Not trusting packet markings
Cisco exams often test conceptual mistakes, not just commands.
Key Exam Takeaways
- A QoS policy controls how traffic is treated
- Built using class maps + policy maps
- Applied using a service policy
- Policies define priority, bandwidth, shaping, policing, and marking
- Works in both wired and wireless networks
- Direction and trust boundaries are critical
- Default class always exists
One-Line Exam Summary
A QoS policy is a set of rules that classifies traffic and defines how network devices prioritize, queue, limit, or mark that traffic in both wired and wireless environments.
