Introduction to BGP Path Selection

Border Gateway Protocol (BGP) is the backbone of internet routing, responsible for exchanging routing information between autonomous systems. However, its path selection process is often misunderstood due to its complexity.

The BGP Decision Process

BGP selects the best path through a multi-step decision process:

1. Highest Weight: Local configuration parameter
2. Highest Local Preference: Internal routing policy
3. Shortest AS Path: Fewer autonomous systems traversed
4. Lowest Origin Type: IGP preferred over EGP
5. Lowest MED (Multi-Exit Discriminator): Preference for entering an AS
6. eBGP over iBGP: External routes preferred
7. Lowest IGP Metric: Shortest path to the NEXT HOP
8. Oldest Route: Tiebreaker for routes with equal attributes

Code Snippet: BGP Configuration Example

router bgp 65000
 neighbor 192.168.1.1 remote-as 65001
 neighbor 192.168.1.1 weight 200
 network 10.0.0.0 mask 255.0.0.0

Practical Implications

Understanding this process is crucial for:

• Network redundancy
• Traffic engineering
• Optimizing route selection

Conclusion

BGP’s path selection is a nuanced process that requires deep understanding of network topology and routing policies.