How RIP Routing Protocol Works

When an IP packet arrives at a router's interface, the router examines the packet’s destination address and finds it in its routing table. Each routing table entry includes two essential things: the destination subnet and the corresponding local interface. If the router finds a matching entry for the destination address, it forwards the packet through the specified interface. If there is no matching entry, the router discards the packet.

Routing table entries can be added either manually or dynamically. In the manual method, you manually add all routes yourself. In the dynamic method, a routing protocol automatically discovers and adds network paths to the routing table. RIP (Routing Information Protocol) is a dynamic routing protocol. Once it is configured, it automatically discovers all network paths and adds them to the routing table without manual intervention.

How does the RIP routing protocol work?

RIP requires information about each router’s directly connected networks. Initially, this information is provided, and RIP is activated on the routers. Once running, each router sends routing updates from all active interfaces every 30 seconds. Each router also receives updates from its neighbors. Each routing update sent by a router includes all of the entries from its routing table, showing the distance to each destination. Neighboring routers then receive these updates and use the information to compare with their own tables and add any new routes they discover. With each subsequent update, routers advertise their updated routing tables. Over time, this process ensures all routers learn about every network route in the topology.

This process can be further clarified through the following example.

Consider a network with two routers, A and B. Once RIP is configured on both, they automatically exchange information about their directly connected networks.

If RIP detects a change in any directly connected network, it informs neighboring routers in the next update. As a result, you only need to provide initial network information. RIP automatically manages all subsequent changes.

RIP Routing broadcasts

To share the paths' information, the RIP protocol uses broadcast messages. RIP protocol periodically reads the routing table and shares it with neighbors through a broadcast message. Upon receiving a broadcast message from a neighbor, the RIP protocol reads it and updates the routing table accordingly. For example, if the broadcast message contains information about a new path, the RIP protocol adds that path to the routing table, or if the broadcast message contains information that an existing path has gone down, the RIP protocol removes that path from the routing table or marks that path unusable in the routing table.

When a router running RIP broadcasts its routing table, it not only announces the locally connected networks but also those learned from its neighbors via previously received broadcasts. This update sequence eventually enables all routers to learn all available paths. For example, consider a network in which four routers A, B, C, and D are connected in sequence. Each router uses the RIP routing protocol. Networks 10.0.0.0/8, 20.0.0.0/8, 30.0.0.0/8, and 40.0.0.0/8 are locally connected to routers A, B, C, and D, respectively.

The routing update sequence proceeds as follows.

• Router A broadcasts network information for 10.0.0.0/8 to Router B.
• Router B broadcasts network information for 20.0.0.0/8 to Router A and Router C.
• Router C broadcasts network information for 30.0.0.0/8 to Router B and Router D.
• Router D broadcasts network information for 40.0.0.0/8 to Router C.

All routers, upon receiving a broadcast update, update their routing tables accordingly.

• Router A adds an entry to the routing table indicating that the network 20.0.0.0/8 is reachable via Router B.
• Router B adds entries to the routing table indicating that networks 10.0.0.0/8 and 30.0.0.0/8 are reachable via Router A and Router C, respectively.
• Router C adds entries to the routing table indicating that networks 40.0.0.0/8 and 20.0.0.0/8 are reachable through Router B and Router D, respectively.
• Router D adds an entry to the routing table indicating that network 30.0.0.0/8 is reachable via Router C.

The following image shows this process.

After the next routing update:

• Router A adds an entry to the routing table indicating that the network 30.0.0.0/8 is reachable via Router B.
• Router B adds an entry to the routing table indicating that the network 40.0.0.0/8 is reachable via Router C.
• Router C adds an entry to the routing table indicating that the network 10.0.0.0/8 is reachable via Router B.
• Router D adds an entry to the routing table indicating that the network 20.0.0.0/8 is reachable via Router C.

The following image shows routing tables before and after the second routing broadcast.

After the next routing update:

• Router A adds an entry to the routing table indicating that the network 40.0.0.0/8 is reachable via Router B.
• Router D adds an entry to the routing table indicating that the network 10.0.0.0/8 is reachable via Router C.

The following image shows the routing tables before and after the third routing broadcast.

The situation in which all routers know all network paths is called convergence. After the convergence, the RIP routing protocol actively monitors all paths. If it detects a change in any path, it updates neighboring routers about it in the next broadcast.

Conclusion

The RIP routing protocol provides a straightforward, automated mechanism for routers to share and update network path information. Its periodic broadcasting and automatic convergence ensure that all routers remain aware of available routes, thereby simplifying network management. Although it is best suited for smaller networks due to its simplicity and inherent limitations, it remains a foundational protocol that exemplifies the core principles of dynamic routing in IP networks.