Border Gateway Protocol

Border Gateway Protocol is the Internet's path-vector routing protocol. It exchanges IP prefix reachability and path attributes so networks can apply administrative policy rather than choosing routes only by physical distance.

Autonomous Systems and Sessions

An autonomous system is a set of networks under a common routing administration, identified for BGP by an autonomous system number. External BGP exchanges routes between autonomous systems; internal BGP distributes BGP reachability within one AS.

BGP peers establish a session over TCP port 179. A working TCP session is only the transport foundation; BGP capabilities, policies, and route exchange must also succeed.

CheckWhat does external BGP exchange?

Ethernet frame checksums within one switch. User passwords between web browsers. Reachability and path information between autonomous systems.

Path-Vector Information

An advertisement includes a prefix and attributes. AS_PATH lists autonomous systems traversed and helps detect loops. Other common attributes include LOCAL_PREF, MED, origin, next hop, and communities. Their effect depends on direction, implementation, and policy.

CheckHow does AS_PATH help prevent inter-AS loops?

An AS can reject a path that already contains its own number. It encrypts every packet crossing those systems. It assigns a MAC address to every AS.

Policy-Based Selection

BGP's “best” path is the path that wins a configured decision process. Operators can prefer customer routes, alter local preference, filter prefixes, use communities, and apply traffic-engineering policy. A shorter AS_PATH can matter at one step but does not universally override higher-priority attributes.

After BGP selects candidates, normal IP forwarding still applies longest-prefix matching. A selected /24 is used for its destinations instead of a selected covering /16.

CheckWhat does a BGP best path represent?

The route that wins the local attribute and policy decision process. The physically shortest cable route in every case. A guarantee of the lowest current application latency.

Advertisement and Reachability

Advertising a prefix asserts reachability under policy; it does not create the underlying route or ensure the return path. Before originating a prefix, ensure valid forwarding, aggregation behavior, filters, failover, and ownership authorization.

CheckWhat does advertising a prefix fail to guarantee?

That peers can receive a control-plane route. That the prefix contains address bits. That it can deliver packets for the whole prefix.

Routing Security and Change Control

Route leaks and hijacks can affect traffic far beyond one router. Operators use strict import and export filters, maximum-prefix limits, peer policy, monitoring, and Resource Public Key Infrastructure origin validation where appropriate. RPKI origin validation checks whether an AS is authorized to originate a prefix; it does not validate the complete AS path.

BGP changes require staged rollout, route-diff review, out-of-band access, rollback, and both control- and data-plane verification.

CheckWhat does RPKI origin validation check?

Whether every packet payload is malware-free. Whether the complete AS path has the lowest latency. Whether the origin AS is authorized.

Summary

You can now describe BGP as policy-controlled path-vector routing.

1. Distinguish external from internal BGP sessions.
2. Use AS_PATH as path and loop information.
3. Interpret best path through local attributes and policy.
4. Verify forwarding behind every advertised prefix.
5. Apply filtering, origin validation, monitoring, and rollback.