QoS Classification and Marking

February 22, 2025

1. Introduction

Quality of Service (QoS) ensures that critical traffic like VoIP, video streaming, and business applications get higher priority over less important traffic like bulk data transfers or web browsing.

Key Concepts

Classification: Identifying and categorizing network traffic.
Marking: Assigning a priority value (like DSCP or CoS) to traffic.

2. QoS Classification

What is Classification?

Classification identifies and groups network traffic based on parameters such as:
✅ Source/Destination IP or Port
✅ Application type (VoIP, Video, Web, FTP)
✅ Protocol (TCP, UDP, ICMP)
✅ Interface (LAN, WAN, VPN)

Traffic Classification Methods

Method	Example
Access Control Lists (ACLs)	Match traffic based on IP, protocol, or port
Class Maps	Define traffic classes in QoS policies
Network-Based Application Recognition (NBAR)	Identifies applications dynamically (Skype, Zoom, etc.)
802.1p (CoS)	Layer 2 VLAN tagging
Differentiated Services Code Point (DSCP)	Layer 3 IP header marking

📌 Example of Traffic Classification Using ACLs:

ip access-list extended VOICE-TRAFFIC
 permit udp any any range 16384 32767

📌 Using Class Maps to Classify Traffic:

class-map MATCH-VOICE
 match access-group name VOICE-TRAFFIC

3. QoS Marking

What is Marking?

Marking assigns priority values to packets so that network devices (routers, switches) can prioritize traffic accordingly.

Common Marking Methods

Marking Type	Layer	Bits	Value Range
802.1p (CoS)	Layer 2 (VLAN)	3 bits	0-7
DSCP (Differentiated Services Code Point)	Layer 3 (IP)	6 bits	0-63
IP Precedence	Layer 3 (IP)	3 bits	0-7

📌 Marking Traffic with DSCP (Layer 3):

class-map MATCH-VOICE
 match ip dscp 46  # Expedited Forwarding (EF) for VoIP

📌 Marking Traffic with CoS (Layer 2 VLAN):

class-map MATCH-VIDEO
 match cos 5  # High priority video

4. DSCP Marking Values

DSCP values define traffic priority levels:

DSCP Class	DSCP Value	Traffic Type
EF (Expedited Forwarding)	46	VoIP (Low Latency)
AF41, AF42, AF43	34, 36, 38	Video Streaming
AF31, AF32, AF33	26, 28, 30	Mission-Critical Apps
AF21, AF22, AF23	18, 20, 22	Bulk Data
CS0 (Default)	0	Best Effort

📌 Example Configuration – Mark VoIP Traffic as EF (DSCP 46):

policy-map MARK-VOICE
 class MATCH-VOICE
  set dscp ef

📌 Trust DSCP on a Switch Port:

interface GigabitEthernet1/0/1
mls qos trust dscp

5. Applying Classification and Marking on an Interface

1️⃣ Create an ACL to Match VoIP Traffic

ip access-list extended VOICE-TRAFFIC
 permit udp any any range 16384 32767

2️⃣ Create a Class Map

class-map MATCH-VOICE
 match access-group name VOICE-TRAFFIC

3️⃣ Create a Policy Map to Mark Traffic

policy-map MARK-VOICE
 class MATCH-VOICE
  set dscp ef

4️⃣ Apply QoS Policy to an Interface

interface GigabitEthernet1/0/1
service-policy input MARK-VOICE

6. Summary

Step	Command
Classify Traffic	`class-map MATCH-VOICE`
Mark DSCP Values	`set dscp ef`
Mark CoS Values	`set cos 5`
Apply to Interface	`service-policy input MARK-VOICE`

Conclusion:

Classification helps identify network traffic.
Marking assigns priority values (DSCP, CoS).
Proper QoS policies ensure that voice, video, and critical data traffic get prioritized over non-essential traffic.

The Lazy Networker