Learning Objectives
- Distinguish between Interior and Exterior Gateway Protocols
- Understand how dynamic routing protocols discover and share routes
- Compare distance vector and link-state protocols
- Know when to use IGP vs EGP
What is Dynamic Routing?
Dynamic routing protocols automatically discover remote networks, share reachability information with neighboring routers, and adapt when the topology changes. If a link goes down, a dynamic protocol recalculates paths — no human intervention required. This is the foundation of the internet's resilience.
Unlike static routing, where every change requires manual entry on every affected router, dynamic routing lets routers "talk" to each other. A router running a dynamic protocol learns routes from its neighbors, propagates those routes to other neighbors, and builds a complete picture of the network topology.
Interior vs Exterior Gateway Protocols
Routing protocols fall into two categories:
| Feature | IGP (Interior Gateway Protocol) | EGP (Exterior Gateway Protocol) | |---------|--------------------------------|--------------------------------| | Scope | Within a single autonomous system | Between autonomous systems | | Examples | RIP, OSPF, EIGRP, IS-IS | BGP | | Design Goal | Fast convergence, rich metrics | Policy control, scalability | | Scale | Hundreds to thousands of routes | Hundreds of thousands to millions | | Convergence | Seconds | Minutes |
An Autonomous System (AS) is a network under a single administrative control. Large organizations, ISPs, and cloud providers each operate one or more AS numbers. IGPs run inside each AS; BGP runs between them.
Distance Vector vs Link-State
IGPs themselves split into two families:
Distance Vector protocols (RIP, EIGRP) know only the direction (vector) and distance to each destination. Each router tells its neighbors what it knows — "I can reach network X with metric Y." This is sometimes called "routing by rumor" because a router trusts what its neighbor reports.
Link-State protocols (OSPF, IS-IS) build a complete map of the network. Every router knows every link, every neighbor, and the cost of each connection. Each router then independently runs the SPF (Shortest Path First) algorithm to compute the best path to every destination.
Match each protocol to its correct category.
Hints
- All IGPs run inside a single AS. BGP is the only EGP.
- Link-state protocols build complete topology maps.
Convergence and Metrics
Convergence is the time it takes for all routers in a network to agree on the best paths after a change. Faster convergence means less downtime. OSPF converges in seconds; BGP can take minutes because it prioritizes stability over speed.
Each protocol uses its own metric:
- RIP: Hop count (max 15)
- OSPF: Cost (based on bandwidth)
- EIGRP: Composite metric (bandwidth, delay, load, reliability)
- BGP: Path attributes (AS path length, local preference, MED)
Which statement best describes the difference between IGPs and EGPs?
Why are dynamic protocols preferred over static routing in large networks?
Key Takeaways
- IGP runs inside an AS; EGP (BGP) runs between ASes
- Distance vector protocols know only direction and distance; link-state protocols know the full topology
- Convergence time varies: OSPF converges in seconds, BGP in minutes
- Protocol choice depends on scale, convergence requirements, and policy needs