RIP

Short for Routing Information Protocol. A distance vector routing protocol. Uses hop count as its sole metric. Runs over UDP port $520$.

Suitable only for small networks. Does not scale beyond $15$ hops.

Infinity and Hop Limit

Infinity is defined as $16$ hops. Any route reaching $16$ is declared unreachable and purged. Maximum usable hop count is $15$.

The cap bounds the count-to-infinity problem: once a failing route reaches $16$, it is removed rather than incrementing indefinitely.

Timers

• Update timer
  $30$ s. Routers broadcast their full routing table to all neighbors every $30$ s.
• Invalid timer
  $180$ s. If no update is received for a route within $180$ s, the route is marked invalid (metric set to $16$).
• Flush timer
  $240$ s. Invalid routes are removed from the table after $240$ s total since the last update.

Versions

RIPv1

Classful. Does not include subnet masks in updates. All routes assumed to use the natural class boundary. Cannot support VLSM or CIDR. Broadcasts updates to 255.255.255.255.

RIPv2

Classless. Includes subnet masks in updates. Supports VLSM and CIDR. Multicasts updates to 224.0.0.9. Supports simple password authentication.

Limitations

• Metric is hop count only. Does not consider bandwidth, delay, or reliability.
• Slow convergence. Full table broadcast every $30$ s; invalid timer adds up to $180$ s of stale route propagation.
• Hard limit of $15$ hops makes it unsuitable for large networks.
• High bandwidth overhead from full table broadcasts.