Traffic Shaping in QoS

1. Introduction

Traffic shaping is a QoS mechanism that controls the rate of outbound traffic to prevent network congestion and packet loss. It smooths traffic bursts by buffering and delaying packets to maintain a steady transmission rate.

📌 Key Benefits of Traffic Shaping: ✅ Prevents network congestion
✅ Ensures consistent bandwidth allocation
✅ Helps with Service Level Agreements (SLAs)
✅ Reduces packet drops in bursty traffic

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2. How Does Traffic Shaping Work?

Traffic shaping buffers excess packets in a queue instead of dropping them, then sends them at a regulated rate.

Shaping vs. Policing

Feature	Shaping	Policing
Purpose	Slows down traffic	Drops or marks excess traffic
Effect on Packets	Buffers packets in a queue	Discards or reclassifies packets
Use Case	WAN links, avoiding congestion	Enforcing strict bandwidth limits
Typical Deployment	Outbound (egress)	Inbound (ingress) & outbound

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3. Configuring Traffic Shaping on Cisco Routers

Cisco uses Class-Based Traffic Shaping (CBTS) to configure shaping per class of traffic.

A. Shaping All Traffic on an Interface

To shape all traffic on an interface to 5 Mbps:

interface Serial0/0/0
traffic-shape rate 5000000

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B. Class-Based Traffic Shaping

1️⃣ Define a Class to Match Traffic:

class-map MATCH-VOICE
 match ip dscp ef

2️⃣ Create a Policy Map to Shape Traffic:

policy-map SHAPE-POLICY
 class MATCH-VOICE
  shape average 1000000

3️⃣ Apply to an Interface:

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

🔹 Explanation:

• Traffic matching DSCP EF (VoIP) is shaped to 1 Mbps.
• Other traffic is not affected unless added to the policy.

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C. Configuring Hierarchical Shaping

For multiple traffic types, use Hierarchical QoS:

policy-map CHILD-POLICY
 class MATCH-VOICE
  priority 500
 class MATCH-VIDEO
  shape average 2000000

policy-map PARENT-POLICY
 class class-default
  shape average 5000000
  service-policy CHILD-POLICY

📌 This means:

• The parent policy shapes all traffic to 5 Mbps.
• The child policy shapes VoIP to 500 Kbps and Video to 2 Mbps.

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4. Verifying Traffic Shaping

✅ To check if shaping is working:

show policy-map interface GigabitEthernet1/0/1

✅ To monitor traffic rates:

show interfaces GigabitEthernet1/0/1 | include rate

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5. Summary

Scenario	Configuration
Shape all traffic to 5 Mbps	traffic-shape rate 5000000
Shape VoIP to 1 Mbps	shape average 1000000
Hierarchical shaping for multiple classes	Parent + Child Policy Maps
Verify shaping	show policy-map interface

🚀 Traffic shaping ensures a smooth and stable network experience by preventing congestion!