The Lazy Networker

1. Introduction

In Quality of Service (QoS), traffic is classified and prioritized to ensure better performance for critical applications. Two common packet marking mechanisms used in QoS are:

1. IP Precedence (Legacy – 3 Bits)
2. Differentiated Services Code Point (DSCP – 6 Bits)

These values are set in the Type of Service (ToS) byte in the IP header to indicate priority levels.

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2. IP Precedence (Legacy)

• 3-bit field (values: 0-7)
• Used in older networks (before DSCP)
• Higher values = Higher priority

IP Precedence	Binary	Priority Level	Traffic Type
7	111	Highest	Network Control
6	110	High	Internetwork Control
5	101	Critical	VoIP, Video
4	100	High	Streaming Media
3	011	Medium	Transactional Traffic
2	010	Normal	Bulk Data
1	001	Low	Scavenger Traffic
0	000	Default	Best Effort

📌 Example Configuration (Mark Traffic with IP Precedence 5 – VoIP):

class-map VOICE
 match ip precedence 5

policy-map QOS-POLICY
 class VOICE
  priority 1000

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3. Differentiated Services Code Point (DSCP)

• 6-bit field (values: 0-63)
• More granular control than IP Precedence
• Backward-compatible with IP Precedence

DSCP Classes

DSCP values are divided into different categories:

1️⃣ Expedited Forwarding (EF – High Priority)

DSCP	Binary	Description
EF (46)	101110	VoIP, real-time apps

🟢 EF (Expedited Forwarding) is used for low-latency traffic like VoIP.

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2️⃣ Assured Forwarding (AF – Prioritized Traffic)

Assured Forwarding (AF) provides four service classes (AF1-4) with three levels of drop probability (Low, Medium, High).

AF Class	DSCP Value	Binary	Drop Probability
AF11	10	001010	Low
AF12	12	001100	Medium
AF13	14	001110	High
AF21	18	010010	Low
AF22	20	010100	Medium
AF23	22	010110	High
AF31	26	011010	Low
AF32	28	011100	Medium
AF33	30	011110	High
AF41	34	100010	Low
AF42	36	100100	Medium
AF43	38	100110	High

🔵 Example Use Case:

• AF41 is used for video streaming.
• AF31 is used for mission-critical applications.

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3️⃣ Default & Best Effort (Low Priority)

DSCP	Binary	Description
CS0 (0)	000000	Best Effort (Default)
CS1 (8)	001000	Background Traffic

🚫 CS0 (Best Effort) is used for general internet traffic without priority.

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4. Mapping Between IP Precedence and DSCP

IP Precedence	Equivalent DSCP
0	CS0 (0)
1	CS1 (8)
2	CS2 (16)
3	CS3 (24)
4	CS4 (32)
5	CS5 (40)
6	CS6 (48)
7	CS7 (56)

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5. Configuring DSCP on Cisco Devices

Marking Packets with DSCP

class-map MATCH-VOICE
 match ip dscp 46

policy-map QOS-POLICY
 class MATCH-VOICE
  priority 1000

Trust DSCP on an Interface

conf t
interface GigabitEthernet1/0/1
mls qos trust dscp
exit

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6. Summary

• IP Precedence (Legacy): 3-bit field (0-7), basic priority levels.
• DSCP (Modern QoS): 6-bit field (0-63), more flexibility.
• EF (46) for VoIP, AF41 for Video, CS0 for Best Effort.