router rip

Overview

The `router rip` command is the gateway to configuring the Routing Information Protocol (RIP) on a Cisco router. RIP is a distance-vector routing protocol that uses hop count as its metric, with a maximum of 15 hops to prevent routing loops. It is one of the oldest routing protocols, defined in RFC 1058 (RIPv1) and RFC 2453 (RIPv2). While RIP is often considered legacy and is rarely used in modern large-scale networks due to its slow convergence and limited scalability, it remains relevant in small networks, lab environments, and for CCNA/CCNP certification purposes. Understanding RIP provides a foundation for learning more advanced protocols like EIGRP and OSPF.

When you enter `router rip` in global configuration mode, the router enters RIP router configuration mode, indicated by a prompt like `Router(config-router)#`. From here, you can enable RIP on specific interfaces using the `network` command, configure version (RIPv1 or RIPv2), adjust timers, enable authentication, and apply route filtering. Without this command, RIP is not active on the router.

The key networking concept behind RIP is that each router periodically sends its entire routing table to neighboring routers (every 30 seconds by default). Routers then update their tables based on received routes, using the Bellman-Ford algorithm to calculate the best path. RIP uses split horizon, route poisoning, and hold-down timers to prevent loops. RIPv2 adds support for VLSM (Variable Length Subnet Mask) and authentication.

You would reach for `router rip` in scenarios where you need a simple, easy-to-configure routing protocol for a small network (e.g., fewer than 15 routers) where bandwidth and convergence time are not critical. Alternatives include static routing (for very small or stub networks), EIGRP (Cisco proprietary, faster convergence, more scalable), or OSPF (open standard, hierarchical, suitable for larger networks). RIP is often used in labs for learning because of its simplicity.

In the broader network configuration workflow, enabling RIP is typically one of the first steps after basic interface IP addressing. The workflow might be: configure interfaces with IP addresses, then enable a routing protocol (like RIP) to exchange routes dynamically. For troubleshooting, `show ip route` and `show ip protocols` are commonly used alongside RIP configuration.

Important IOS behavior: The `router rip` command does not require any special privilege level beyond global config (privilege level 15). It immediately enters RIP configuration mode. The running config will show `router rip` and any subsequent RIP commands. If you exit without configuring any networks, RIP will be enabled but will not advertise any routes. Also, RIP is a classful protocol by default (RIPv1), so it will automatically summarize at classful boundaries unless you disable auto-summary. The command `no router rip` removes the entire RIP configuration.

RIP updates are sent to UDP port 520. The router listens on this port and sends updates from all interfaces that have an IP address matching a network statement. By default, RIP sends updates to the broadcast address 255.255.255.255 (RIPv1) or multicast 224.0.0.9 (RIPv2).