🗺️ Network Topology Example

We have 4 Autonomous Systems (ASes):

1
2
3
+-------+        +-------+        +-------+        +-------+
| AS 100| <----> | AS 200| <----> | AS 300| <----> | AS 400|
+-------+        +-------+        +-------+        +-------+

Let’s say:

• AS 400 originates a route to network 10.0.0.0/24

• It announces:

  1
  "To reach 10.0.0.0/24, go through [AS400]"
  

🔁 How the Path Vector Propagates

1. AS 300 receives it and appends its own AS:

   1
   "10.0.0.0/24 via [AS300, AS400]"
   

2. AS 200 receives it:

   1
   "10.0.0.0/24 via [AS200, AS300, AS400]"
   

3. AS 100 receives it:

   1
   "10.0.0.0/24 via [AS100, AS200, AS300, AS400]"
   

🔁 What If a Loop Tries to Form?

Let’s say now AS 400 receives the same route back from AS 100. That route would look like:

1
"10.0.0.0/24 via [AS100, AS200, AS300, AS400]"

🛑 But AS 400 sees its own AS number (AS400) in the path.

✅ So it rejects the route — loop avoided!

🧠 Why This Works

• The path vector protocol includes the entire route history (AS_PATH).
• A router can look at the path and detect loops before they happen.
• Unlike RIP (which might loop until hop count hits 16), BGP can instantly discard bad routes.

✅ Summary: How Path Vector Prevents Loops