1.5 Interpret wired and wireless QoS configurations
📘CCNP Encore (350-401-ENCORE-v1.1)
What is QoS (Quality of Service)?
Quality of Service (QoS) is a set of technologies used in networks to control, prioritize, and manage traffic so that important applications (like voice, video, and critical business apps) get the performance they need.
QoS helps the network decide:
- Which traffic is more important
- Which traffic can wait
- Which traffic should be limited or dropped during congestion
QoS is used in:
- Wired networks (switches, routers)
- Wireless networks (Wi-Fi controllers, access points)
Why QoS Is Needed
Not all network traffic is equal:
- Voice and video need low delay and low packet loss
- File downloads and backups can wait
- Business-critical apps must be protected
Without QoS:
- Voice calls may sound broken
- Video may freeze
- Important applications may become slow
Main QoS Components (Exam Focus)
For the CCNP ENCOR exam, QoS is built around five core components:
- Classification
- Marking
- Congestion Management (Queuing)
- Congestion Avoidance
- Policing and Shaping
You must understand what each component does, where it is used, and how they work together.
1. Classification
What is Classification?
Classification is the process of identifying and grouping traffic into different classes.
The network examines packets and decides:
“What type of traffic is this?”
How Traffic Is Classified
Traffic can be classified based on:
- Source or destination IP address
- Protocol (TCP, UDP)
- Port numbers (e.g., voice, video, applications)
- VLAN
- DSCP or CoS values (if already marked)
Why Classification Is Important
Before the network can:
- Prioritize traffic
- Queue traffic
- Limit traffic
It must first recognize the traffic type.
➡️ Classification is always the first step in QoS
Exam Tip
- Classification happens at the network edge (access switch, wireless controller)
- QoS decisions depend on correct classification
2. Marking
What is Marking?
Marking means labeling packets so that other network devices know how to treat them.
A marked packet carries priority information as it travels through the network.
Why Marking Is Needed
Networks are made of many devices:
- Switches
- Routers
- Wireless controllers
Marking ensures:
- QoS decisions are consistent
- Devices don’t need to re-classify traffic again
Common QoS Marking Types (Important for Exam)
DSCP (Differentiated Services Code Point)
- Used at Layer 3 (IP layer)
- Most common and most important for CCNP
- Values range from 0 to 63
Examples:
- EF (Expedited Forwarding) → Voice traffic
- AF (Assured Forwarding) → Business apps
- BE (Best Effort) → Default traffic
CoS (Class of Service)
- Used at Layer 2 (Ethernet/VLAN)
- 3-bit value (0–7)
- Only works inside the LAN
Exam Tip
- DSCP is trusted end-to-end
- CoS is only valid inside VLANs
- Marking should be done as close to the source as possible
3. Congestion Management (Queuing)
What is Congestion?
Congestion happens when:
- More traffic arrives than the interface can send
- Packets start waiting or getting dropped
What is Queuing?
Queuing is how the network:
- Stores packets temporarily
- Decides which packets are sent first
Why Queuing Is Important
When congestion occurs:
- Important traffic should go first
- Less important traffic should wait
Queuing ensures:
- Priority traffic gets better performance
- Fairness between traffic classes
Types of Queues (Conceptual – Exam Level)
- Priority Queue
- Used for delay-sensitive traffic
- Sent before other queues
- Class-based Queues
- Traffic is divided into classes
- Each class gets a portion of bandwidth
Exam Tip
- Queuing does not increase bandwidth
- It only controls packet order during congestion
4. Congestion Avoidance
What is Congestion Avoidance?
Congestion avoidance tries to:
- Prevent congestion from getting worse
- Avoid sudden packet drops
How Congestion Avoidance Works
Instead of dropping packets randomly:
- The network starts dropping packets early
- This signals sending devices to slow down
This is especially important for:
- TCP-based applications
Key Concept
- Congestion avoidance protects queues from filling completely
- It improves overall network stability
Exam Tip
- Congestion avoidance works before queues are full
- It helps avoid global TCP slowdowns
5. Policing and Shaping
These components control traffic rates.
5.1 Policing
What is Policing?
Policing enforces a hard traffic limit.
If traffic exceeds the limit:
- Packets are dropped or re-marked
Where Policing Is Used
- Network edges
- WAN links
- Service provider connections
Key Characteristics
- Immediate enforcement
- No buffering
- Excess traffic is removed
Exam Tip
- Policing is strict
- Used to protect network resources
5.2 Shaping
What is Shaping?
Shaping controls traffic by:
- Delaying excess packets
- Sending them later when bandwidth is available
Where Shaping Is Used
- WAN interfaces
- Slow links
- When smooth traffic flow is required
Key Characteristics
- Uses buffering
- Traffic is delayed, not dropped
- Produces smoother traffic patterns
Policing vs Shaping (Very Important for Exam)
| Feature | Policing | Shaping |
|---|---|---|
| Excess traffic | Dropped or re-marked | Delayed |
| Buffering | No | Yes |
| Strictness | High | Soft |
| Common use | Provider edge | Customer WAN |
QoS in Wireless Networks (High-Level)
Wireless QoS uses the same core principles:
- Classification
- Marking
- Prioritization
But also includes:
- Traffic categories (voice, video, best effort, background)
- Airtime fairness
➡️ Wireless QoS ensures:
- Voice and video are protected over Wi-Fi
- Wireless congestion is managed efficiently
End-to-End QoS Flow (Easy to Remember)
- Classify traffic
- Mark traffic
- Queue traffic during congestion
- Avoid congestion early
- Police or shape traffic if needed
Key Exam Takeaways
- QoS is about managing traffic, not adding bandwidth
- Classification and marking are foundational
- Queuing handles congestion
- Policing drops traffic, shaping delays traffic
- DSCP is the most important marking method
- QoS applies to both wired and wireless networks
