Routing protocols fall into categories based on where they operate.
Interior Gateway Protocols (IGPs) run inside a single organization:
- OSPF, IS-IS, EIGRP, RIP
- Share detailed topology information
- Optimize for fastest path
- Trust all participating routers
Exterior Gateway Protocols (EGPs) run between organizations:
- BGP is the only EGP used today
- Share reachability information, not topology
- Optimize for policy compliance
- Treat neighbors with caution
You need different approaches because organizations don't share internal details. Your ISP won't tell competitors about its backbone topology.