A routing protocol is a set of rules that routers use to share information about networks and choose the best paths automatically. Without routing protocols, administrators would have to type every route by hand and update them whenever a link failed. This guide explains how routing protocols work and compares the main types.
What a routing protocol does
- Discovers neighbours: routers find other routers running the same protocol.
- Exchanges routing information: they share the networks they know.
- Calculates best paths: each router uses a metric to pick the best route.
- Updates the routing table: the best routes are installed for forwarding.
- Reacts to change: when a link fails, routers find an alternative path.
Routing protocol vs routed protocol
A routed protocol, such as IPv4 or IPv6, carries user data and has addresses that routers can forward. A routing protocol, such as OSPF or BGP, carries information *about* routes. In other words, routing protocols build the map, while routed protocols travel on it.
Main types of routing protocols
Distance vector
Each router knows only what its neighbours tell it: the destination and the distance. It is sometimes called “routing by rumour”.
- Example: RIP.
- Pros: simple and light.
- Cons: slow convergence and risk of loops, which features like split horizon reduce.
Read more about RIP.
Link state
Every router builds a complete map of its area from link-state advertisements, then runs Dijkstra’s shortest path first algorithm.
- Examples: OSPF, IS-IS.
- Pros: fast convergence, accurate paths, scales with areas.
- Cons: more memory and CPU, more design work.
See our guide to OSPF.
Advanced distance vector (hybrid)
This type keeps distance vector simplicity but adds fast, loop-free convergence.
- Example: EIGRP, using the DUAL algorithm.
See EIGRP.
Path vector
Routes carry the full list of autonomous systems they pass through. Routers use this path and policies to choose routes between organizations.
- Example: BGP, the protocol of the internet. See what is BGP.
Comparison
| Protocol | Type | Metric | Scope | Convergence |
|---|---|---|---|---|
| RIP | Distance vector | Hop count | Small networks | Slow |
| EIGRP | Advanced distance vector | Bandwidth and delay | Enterprise (mainly Cisco) | Very fast |
| OSPF | Link state | Cost | Enterprise and ISP | Fast |
| IS-IS | Link state | Configurable metric | Large ISPs | Fast |
| BGP | Path vector | Path attributes and policy | Between autonomous systems | Slower, policy-driven |
Key concepts
- Convergence: the time until all routers agree on the network’s state after a change. Faster convergence means shorter outages.
- Classful vs classless: classless protocols send subnet masks, so they support VLSM and CIDR. All modern protocols are classless.
- Interior vs exterior: IGPs run inside one organization, while BGP runs between them. Learn more in routing protocol categories.
- Administrative distance: decides which protocol wins when several offer the same route.
Choosing a routing protocol
- Small, simple network: static routes or OSPF in a single area.
- Multi-vendor enterprise: OSPF.
- Cisco-only network wanting simple setup: EIGRP.
- Internet connectivity with several ISPs: BGP.
Sources
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