RIP Tutorial – Basic operation of RIP Protocol

There are two versions of the RIP routing protocol: RIPv1 and RIPv2. RIPv1 was one of the earliest and most widely adopted IP routing protocols, commonly used until the 1990s. However, RIPv1 is now considered obsolete in modern networks. RIPv2 was developed in the 1990s as an enhancement of RIPv1 to better meet the needs of evolving networks. It introduced improvements to existing features and added several new capabilities. This tutorial examines the primary differences between RIPv1 and RIPv2, emphasizing their respective features and enhancements. It further explains RIP timers and their role in efficient routing protocol management.

Comparing RIPv1 and RIPv2 routing protocols

• RIPv1 is a distance-vector routing protocol. While RIPv2 is based on RIPv1 and remains a distance-vector protocol at its core, it incorporates additional features that lead some to classify it as a hybrid routing protocol.
• To find and select the best route for each IP subnet in the network, both versions use the hop-count metric. The maximum hop count limit is set to 15 on both versions.
• RIPv1 uses broadcast messages to advertise routing updates, whereas RIPv2 utilizes the multicast address 224.0.0.9 for this purpose. This enhancement conserves network bandwidth.
• RIPv1 does not support triggered updates, whereas RIPv2 does. Triggered updates allow RIPv2 to immediately notify connected neighbors of changes, while RIPv1 only transmits updates during scheduled broadcasts. This capability enables faster network convergence.
• RIPv1 is a classful routing protocol and does not support variable-length subnet masking (VLSM). In contrast, RIPv2 is classless and fully supports VLSM. This capability permits the use of multiple subnet masks within the same network class, thereby maximizing IP address efficiency and facilitating the creation of large, scalable networks through route summarization.
• RIPv1 lacks authentication features. In contrast, RIPv2 supports MD5 authentication, enabling network administrators to restrict which routers exchange routing information and thereby enhance network security.
• Both RIPv1 and RIPv2 employ split horizon, route poisoning, and various timers in their core operations.

RIP timers

RIP uses four timers: the update timer, the route invalid timer, the route flash timer, and the hold-down timer.

The update timer

RIP uses the update timer to define the interval between consecutive routing updates. The default value is 30 seconds. After sending a routing update, RIP waits 30 seconds before transmitting the next update.

The route invalid timer

RIP acquires new routes from received routing updates. When RIP adds these routes to the routing table, it associates an invalid timer with each route. This timer removes routes that become invalid over time. In upcoming routing updates, if RIP receives the same information on a route, it resets the invalid timer of that route in the routing table. If it does not receive the same information about a route within 180 seconds, it assumes the route is no longer available. When it happens, RIP sends an update to all its neighbors to indicate that the route is invalid.

The route flush timer

RIP uses the route flush timer to remove invalid routes from the routing table. Its value must exceed that of the invalid timer, providing sufficient time for RIP to notify neighbors of the invalid route before it is removed. The default value is 240 seconds.

The hold-down timer

RIP uses the hold-down timer to rapidly identify invalid routes. Upon receiving a routing update indicating an invalid route, it immediately initiates the hold-down timer for that route in the routing table. The default value is 180 seconds. RIP stops and removes the hold-down timer from an invalid route only if it receives a routing update with a better metric for that route. If no better metric is received before the hold-down timer expires, it continues to advertise the invalid route to its neighbors until the flush timer expires. The invalid route is then removed once the flush timer expires.

Both the route invalid timer and hold-down timer function similarly, differing primarily in their triggering conditions. The invalid timer is activated when RIP does not receive a routing update for a route within 180 seconds. The hold-down timer is triggered when RIP receives a routing update indicating that a route has become invalid.

Conclusion

This tutorial has highlighted the evolution from RIPv1 to RIPv2, noting the key improvements that make RIPv2 better suited to modern networks. It also explained the crucial role of RIP timers in maintaining efficient routing operations. Understanding these differences and mechanisms is essential for effective network design and management.