Network routing protocols are algorithms and communication standards that allow routers to dynamically discover network paths, share routing information, and make forwarding decisions across interconnected networks. They operate at Layer 3 of the OSI model and form the intelligence layer that enables packets to traverse complex network topologies efficiently. Routing protocols are classified into interior gateway protocols (IGPs) for intra-domain routing within autonomous systems, and exterior gateway protocols (EGPs) for inter-domain routing between autonomous systems. A critical distinction: administrative distance determines which protocol's route wins when multiple protocols advertise the same destination, while metrics within each protocol determine the best path among routes learned by that protocol alone.
What This Cheat Sheet Covers
This topic spans 23 focused tables and 145 indexed concepts. Below is a complete table-by-table outline of this topic, spanning foundational concepts through advanced details.
Table 1: Protocol Classification and Characteristics
Before any specific protocol makes sense, it helps to know which family it belongs to. Routing protocols sort along three axes — where they run (interior versus exterior), how they learn the network (distance vector, link state, or path vector), and whether they carry a subnet mask (classful versus classless) — and those labels predict most of how a protocol behaves.
| Protocol | Example | Description |
|---|---|---|
OSPF, EIGRP, RIP, IS-IS | • Routing protocols designed to operate within a single autonomous system (AS) • focus on fast convergence and path optimization for internal networks | |
BGP | • Routing protocol used between autonomous systems on the internet • emphasizes policy control and path manipulation over speed | |
RIP, EIGRP | • Routers share their entire routing table with directly connected neighbors • decisions based on hop count or composite metrics without full topology visibility | |
OSPF, IS-IS | • Routers flood link state advertisements (LSAs) throughout the area • each router builds identical topology database and runs SPF algorithm independently |