# What is Border Gateway Protocol (BGP)?

> Border Gateway Protocol (BGP) is the routing protocol used to exchange network reachability information between autonomous systems (ASes), such as internet service providers, cloud networks, and large organizations.

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## Content

BGP advertisements identify reachable address prefixes and carry path attributes that help each AS apply its own routing policy and select routes.

External BGP exchanges routes between ASes; internal BGP distributes routes inside an AS. Peers establish a TCP session and then send incremental updates. Selection is policy-driven: the chosen path is not necessarily the geographically shortest, least congested, or most secure path.

### Key points

- **Route information:** Network Layer Reachability Information identifies prefixes, while attributes such as AS\_PATH support selection and loop detection.
- **Routing policy:** Operators control which routes they accept, prefer, and advertise according to business, engineering, and security requirements.
- **Operational protection:** Prefix and AS-path filters, maximum-prefix limits, controlled peering, session protection, monitoring, and change discipline reduce avoidable failures and abuse.
- **Important limitation:** Base BGP does not establish that every announcement is authorized or operationally correct. Resource Public Key Infrastructure (RPKI) route-origin validation can check whether an origin AS is authorized for a prefix, but it does not validate the complete AS path or prevent every route leak.

### Related terms

[TCP/IP protocol suite](<https://yellowcube.eu/glossary/tcp-ip-protocol-suite/>) · [Wide area network (WAN)](<https://yellowcube.eu/glossary/wide-area-network/>) · [IP addressing: static and dynamic addresses](<https://yellowcube.eu/glossary/ip-addressing-static-and-dynamic-addresses/>) · [Network access control list (ACL)](<https://yellowcube.eu/glossary/network-access-control-list/>) · [Network traffic](<https://yellowcube.eu/glossary/network-traffic/>)

### Sources

[IETF RFC 4271: Border Gateway Protocol 4](https://datatracker.ietf.org/doc/html/rfc4271) · [IETF RFC 7454: BGP Operations and Security](https://datatracker.ietf.org/doc/html/rfc7454) · [IETF RFC 6811: BGP Prefix Origin Validation](https://datatracker.ietf.org/doc/html/rfc6811) · [IETF RFC 9582: RPKI Route Origin Authorizations](https://datatracker.ietf.org/doc/html/rfc9582)

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