border gateway protocol network layer computer networks

1. BGP - Border Gateway Protocol
What is BGP?
Border Gateway Protocol (BGP): The protocol used for routing between different Autonomous
Systems (ASes) on the Internet.
Purpose: Ensures that data packets are delivered efficiently across vast networks.
Key Feature: Allows enforcement of routing policies based on politics, economics, or security.
Visual: A diagram showing multiple ASes (AS1, AS2, etc.) connected via BGP links.

2. Why Do We Need BGP?
Intradomain Protocols: (e.g., OSPF, IS-IS) focus on efficiency within a single AS.
Interdomain Protocols: (e.g., BGP) handle:
• Political considerations
• Economic constraints
• Security policies
Example: "Do not route traffic from the Pentagon through an AS in a rival country."
Visual: Two ASes exchanging data, one with a "restricted" route.

3. Key Concepts in BGP
1. Autonomous System (AS): Independent networks managed by organizations (e.g., ISPs,
corporations).
2. Transit: AS provides routes for other ASes (paid service).
3. Peering: ASes exchange traffic directly (usually free).
4. Routing Policies: Rules governing how traffic flows.
5. Visual: Simple flow diagram showing Transit and Peering relationships between AS1, AS2, and
AS3.

4. How BGP Works
• Path Vector Protocol:
Keeps track of the route (AS path) data takes.
• Policies First: Routes are selected based on policies, not just shortest paths.
• TCP-based Communication: Reliable route advertisements between routers.
• Visual: A route advertisement showing AS3 -> AS2 -> AS1.

5. Example Scenario
Transit: AS1 (a provider) advertises routes to AS2, AS3, and AS4. AS2, AS3, and AS4 pay AS1 for
transit services.
Peering: AS2 and AS3 exchange traffic directly to reduce transit costs.
Visual:
A network diagram with AS1 providing transit and AS2-AS3 peering directly.

6. Route Advertisement
AS3 advertises a route to "Prefix C" to AS2.
AS2 appends its AS number and advertises it to AS1.
AS1 propagates the route to its customers.
Visual: Sequential diagram showing the route "C -> AS3 -> AS2 -> AS1."

7. BGP Routing Policies
• Political: Avoid routes through certain regions.
• Economic: Prefer cheaper ISPs.
• Security: Route only through trusted ASes.
Example: "Do not use AT&T in Australia due to poor performance."
Visual: Table showing policy examples.

8. Advantages of BGP
• Scalable for the global Internet.
• Flexible to support various policies.
• Reliable communication via TCP.
Visual: A global map highlighting interconnected networks.

9. Conclusion
BGP enables inter-AS routing and supports routing policies.
It balances performance, security, and cost considerations.
BGP ensures the Internet stays connected globally.
Visual: A simple diagram showing data flowing seamlessly across the globe.