BGP for Beginners: The Protocol That Runs the Internet
BGP (Border Gateway Protocol) is the path-vector protocol that runs the internet. It exchanges routing information between large networks called autonomous systems (AS), deciding how traffic flows between organisations and ISPs worldwide.
Autonomous systems and why BGP exists
The internet is a network of networks. Each large network (an ISP, a big company) is an autonomous system with a unique AS number. BGP is how these systems advertise which destinations they can reach and choose paths between each other.
eBGP vs iBGP
- eBGP — between routers in different autonomous systems (e.g., your ISP and another ISP).
- iBGP — between routers inside the same autonomous system, to carry external routes across the network.
How BGP chooses the best path
Unlike OSPF (which uses cost), BGP evaluates a series of path attributes in order. A simplified decision order is:
- Highest Weight (Cisco-only, local)
- Highest Local Preference
- Shortest AS-Path
- Lowest Origin type
- Lowest MED
This lets network operators shape traffic based on business policy, not just shortest distance.
Where you'll meet BGP
BGP appears in ISP networks, data centres and multi-homed enterprises. It builds on routing fundamentals you learn in CCNA and is covered in depth in CCNP. Compare it with OSPF.
Frequently asked questions
Is BGP in the CCNA exam?
BGP is introduced at a conceptual level; deep BGP configuration is a CCNP/CCIE topic.
What is the difference between eBGP and iBGP?
eBGP runs between different autonomous systems; iBGP runs between routers within the same AS.
Why is BGP called a path-vector protocol?
Because it makes routing decisions based on the path of autonomous systems (AS-path) a route has traversed, plus other attributes.
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