Weighted Random Early Detection (WRED) in QoS

1. Introduction

Weighted Random Early Detection (WRED) is a congestion avoidance QoS mechanism that randomly drops packets before the queue is full, helping to prevent global TCP synchronization and optimize network performance.

📌 Key Benefits of WRED:
✅ Prevents queue tail drops, reducing congestion
✅ Improves TCP traffic performance
✅ Differentiates traffic by priority levels
✅ Works best in TCP-based networks

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2. How WRED Works

WRED monitors queue depth and randomly drops packets based on:

• Queue occupancy (how full the queue is)
• Packet marking (DSCP/IP Precedence)
• Minimum and maximum drop thresholds

🔹 WRED Drop Logic:

Queue Status	Behavior
Queue < Min Threshold	No drops ✅
Queue between Min & Max	Random drops based on priority 🎲
Queue > Max Threshold	All new packets dropped (Tail Drop) 🚫

📌 Higher-priority traffic has higher thresholds, reducing drop probability.

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3. WRED vs. Tail Drop

Feature	WRED	Tail Drop
Packet Drop Behavior	Gradual, based on queue depth	Drops all packets when full
Effect on TCP	Prevents global TCP synchronization	Causes synchronization issues
QoS Differentiation	Prioritizes important traffic	No differentiation

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4. Configuring WRED on a Cisco Router

A. Basic WRED Configuration

To enable WRED on an interface:

interface GigabitEthernet1/0/1
 random-detect

📌 This applies default WRED settings, treating all traffic equally.

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B. WRED with IP Precedence-Based Drop Probabilities

To prioritize higher-priority traffic:

interface GigabitEthernet1/0/1
 random-detect precedence 3 20 40
 random-detect precedence 5 30 60

📌 Explanation:

• Precedence 3 packets → Start dropping at 20% queue, full drop at 40%.
• Precedence 5 packets → Start dropping at 30%, full drop at 60%.
• Higher precedence = Less chance of being dropped.

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C. WRED with DSCP-Based Drop Probabilities

To configure WRED based on DSCP values:

interface GigabitEthernet1/0/1
 random-detect dscp-based

Then, define DSCP drop thresholds:

• DSCP 10 (low priority) → Drops start at 20%, full drop at 50% queue depth.
• DSCP 46 (VoIP, high priority) → Drops start at 40%, full drop at 70%.

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D. WRED with Class-Based QoS

1️⃣ Define a Traffic Class:

class-map MATCH-VIDEO
 match ip dscp af41

2️⃣ Create a Policy with WRED:

policy-map WRED-POLICY
 class MATCH-VIDEO
  random-detect dscp-based

3️⃣ Apply Policy to an Interface:

interface GigabitEthernet1/0/1
 service-policy output WRED-POLICY

📌 Now, WRED is applied only to DSCP AF41 traffic!

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5. Verifying WRED

✅ Check WRED settings on an interface:

show interfaces GigabitEthernet1/0/1 random-detect

✅ Monitor packet drops with WRED:

show policy-map interface GigabitEthernet1/0/1

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6. Summary

Scenario	Configuration
Enable WRED on an interface	random-detect
Prioritize traffic based on IP Precedence	random-detect precedence X min max
Prioritize traffic based on DSCP	random-detect dscp-based
Class-Based WRED	policy-map WRED-POLICY + random-detect dscp-based
Verify WRED settings	show interfaces random-detect

🚀 WRED improves congestion management by preventing queue overflows and TCP synchronization issues!