How BGP Prevents Loops in iBGP and eBGP: A Beginner-Friendly Guide with Scenarios
When we talk about Border Gateway Protocol (BGP), we’re really talking about the glue that holds the internet together. BGP decides how traffic flows between massive networks called Autonomous Systems (AS).
But here’s the catch: with so many routers exchanging routes, what if those routes just kept bouncing in circles forever? That’s what we call a routing loop — and it would completely break the internet.
Luckily, BGP has smart built-in mechanisms to prevent this problem. The way BGP stops loops is a little different depending on whether it’s running between ASes (eBGP) or within the same AS (iBGP). Let’s break it down with simple scenarios.
🔹 Loop Prevention in eBGP
Scenario: What Could Go Wrong
Imagine three autonomous systems — AS100, AS200, and AS300 — connected in a ring.
- AS100 R1 router sends a route (1.1.1.1/32) to R2 AS200.
- The router R2 AS200 passes it to router R3 AS300.
- Without protection, AS300 might send that same route back to AS100.
Now AS100 thinks there’s another path to the same network (through itself), which is nonsense. That’s a loop.
How BGP Fixes This in eBGP
1. AS_PATH Attribute
Every time a route crosses into a new AS, BGP adds that AS number into the AS_PATH list. Think of it as leaving footprints behind.
2. Loop Check
When a router receives a route, it looks at the AS_PATH.
- If its own AS number is already in the list → it means the route came full circle.
- The router rejects the route immediately.
Example:
If Router R1 (AS100) sees a route with AS_PATH = AS300 AS200 AS100, it drops it — because it already sees “AS100” in the path.
This simple trick prevents endless cycling between autonomous systems.
🔹 Loop Prevention in iBGP
Now, let’s zoom inside one AS.
Scenario: Why Loops Can Still Happen
Inside AS100, you might have multiple routers (R1, R2, R3) speaking iBGP. Here’s the problem:
- When a route is shared inside the same AS, the AS_PATH doesn’t grow (because it’s still within AS100).
- That means the AS_PATH trick no longer works for loop detection.
So BGP needs a different method here.
How BGP Fixes This in iBGP
1. Split-Horizon Rule
A route learned from one iBGP neighbor is never re-advertised to another iBGP neighbor.
- If R1 learns a route from R2 via iBGP, it won’t forward it to R3 via iBGP.
- It will only share it with eBGP peers.
This prevents loops but introduces another problem → not all routers may see every route unless they directly peer with each other.
🔹 Summary Table of BGP Loop Prevention
| Feature | eBGP Loop Prevention | iBGP Loop Prevention |
| Main Mechanism | AS_PATH check (reject if own AS present) | Split-Horizon Rule (don’t forward iBGP routes to iBGP) |
| Scenario Addressed | Prevent loops between different ASes | Prevent loops inside the same AS |
| Scaling Solution | N/A | Full Mesh, Route Reflectors, Confederations |
| Example | AS100 drops a route if AS_PATH = “AS100 …” | Router R2 won’t forward iBGP-learned route to Router R3 |
Final Thoughts
BGP may look complex, but its loop prevention is elegant:
- Between ASes → AS_PATH footprints stop routes from circling back.
- Inside an AS → Split-Horizon prevent endless bouncing.
Without these mechanisms, the internet would collapse under its own weight of loops. Next time you troubleshoot BGP, remember these simple rules — they’re the silent guardians keeping your data on the right path.