“AS-Path Prepend Deep Dive for both Inbound & outbound traffic When and Why It Works”

AS-Path Prepend Deep Dive | Inbound vs Outbound Traffic Explained with Real Examples

⏱ Suggested Video Length

12–15 minutes

🧠 INTRO (0:00 – 1:00)

In this video, we’ll do a deep dive into AS-Path Prepending and answer the most confusing question in BGP:

👉 Does AS-Path prepend control inbound traffic, outbound traffic, or both?

We’ll cover:

What AS-Path prepend really does

Why it is mainly used for inbound traffic engineering

Why it does NOT truly control outbound traffic

Real-world CLI examples

When prepend fails and what to use instead

🔍 WHAT IS AS-PATH PREPEND? (1:00 – 2:30)

In BGP, AS-Path is the list of Autonomous Systems a route has traversed.

BGP prefers:

Shortest AS-Path

Before considering MED, origin, or IGP metric

AS-Path prepend means:

Artificially adding your own AS number multiple times to make a path look longer and therefore less attractive.

Example:

Original AS-Path: 65001
After prepend: 65001 65001 65001

🚦 BGP PATH SELECTION (Quick Reminder) (2:30 – 3:30)

Key BGP decision steps (simplified):

Highest Weight (Cisco only)

Highest Local Preference

Shortest AS-Path ← prepend affects this

Origin

MED

eBGP over iBGP

IGP metric to next-hop

⚠️ Important:

AS-Path is evaluated only after Local Preference

If Local Preference is different, prepend will be ignored

📥 AS-PATH PREPEND FOR INBOUND TRAFFIC (3:30 – 7:00)
🔑 THIS IS THE PRIMARY USE CASE

Inbound traffic =
Traffic coming FROM the internet or external AS INTO your network

Scenario:

You have two ISPs

ISP-A (Primary)

ISP-B (Backup)

You want incoming traffic to prefer ISP-A

Solution: