What is BGP, and why is it called the routing protocol of the internet?
BGP (Border Gateway Protocol) exchanges routes between autonomous systems, the independently run networks that make up the internet, such as ISPs and large companies. It chooses paths by policy and AS path length, not simply the shortest link.
Explain it simply
The internet is thousands of separate road authorities. BGP is how they tell neighbours "I can reach these addresses, and here is the list of authorities the traffic passes through". Each chooses routes based on business agreements as much as distance.
Example
AS = autonomous system, identified by an AS number
AS 100 (your ISP) tells AS 200: "I can reach 203.0.113.0/24 via AS path [100]"
AS 200 passes it on: "reach it via AS path [200, 100]"
Path selection (simplified): local preference (policy) first,
then shortest AS path, then other tie-breakers.BGP runs over TCP port 179. Between different autonomous systems it is called eBGP; inside one AS, iBGP.
Routing by trust is its weak point: if a network wrongly announces prefixes it does not own, traffic can be pulled toward it. This route hijacking or leak has caused real outages. Fixes include RPKI, which lets a prefix owner publish which AS may announce it, and route filtering.
What to say in the interview
"BGP is the path-vector protocol that connects autonomous systems. It is policy-driven, runs over TCP 179, and holds the global routing table together. Its main weakness is that it trusts announcements, which RPKI and filters try to fix."
Common mistakes
- Calling BGP a shortest-path protocol. Policy usually decides first.