Lec31-How to construct forwarding tables for Routers in RIP

Key Concepts

• Autonomous System
• Routing Information Protocol (RIP)
• Forwarding Table (vs. Routing Table)
• Initial Forwarding Table
• Final Forwarding Table
• Hop Count
• Route Stabilization
• Route Update Conditions (New Route, Lower Cost, Unreachable)
• Count to Infinity Problem

Final Forwarding Table Construction

The video focuses on constructing final forwarding tables for routers within an autonomous system using the Routing Information Protocol (RIP). The key difference between initial and final forwarding tables is that initial tables only contain information about directly connected networks, while final tables reflect the stabilized state after routing information exchange.

Steps for Constructing Final Forwarding Tables:

1. Identify all networks: Determine the total number of networks within the autonomous system.
2. Create table structure: For each router, create a table with columns for Destination Network, Next Router, and Cost (Hop Count). List all networks as potential destinations.
3. Directly connected networks: For each router, identify the networks to which it is directly connected. The cost to reach these networks is 1, and the "Next Router" field is left blank (or indicated with a dash).
4. Indirectly connected networks: For each remaining network, determine the shortest path (in hops) from the current router to that network.
   • Identify the next router along that shortest path.
   • Record the next router and the total hop count in the forwarding table.
5. Repeat for all routers: Repeat steps 3 and 4 for each router in the autonomous system.

Example: Router R1

• Networks: N1, N2, N3, N4, N5, N6
• Directly connected: N1, N2, N3 (Cost = 1, Next Router = -)
• N4: Shortest path is via R2 (Cost = 2, Next Router = R2)
• N5: Shortest path is via R2 (Cost = 2, Next Router = R2)
• N6: Shortest path is via R2 (Cost = 3, Next Router = R2)

Route Update Conditions

A router updates its forwarding table under three specific conditions:

1. New Route: If the router doesn't have any existing information about a particular network, it will add the new route received from a neighbor to its forwarding table. This ensures that all routers eventually have a route to every network in the autonomous system.
2. Lower Cost: If the router receives a route to a network with a lower cost (hop count) than the existing route, it will replace the old route with the new one. This optimizes routing by selecting the shortest paths. The next router will also be different in this case.
3. Unreachable (Cost = 16): If the router receives a route with a cost of 16, it indicates that the network is unreachable. The router will replace the existing route with the unreachable route only if the next router in both the old and new routes is the same. This is crucial to prevent the "count to infinity" problem.

Explanation of Cost = 16:

• RIP has a maximum hop count of 15. A cost of 16 signifies that a network is unreachable.
• Failing to accept a route with a cost of 16 can lead to routing loops and the "count to infinity" problem, where routers continuously increment the hop count in an attempt to reach an unreachable network.

"Count to Infinity" Problem:

The video alludes to the "count to infinity" problem, where a router keeps increasing the hop count to a network it cannot reach, creating a routing loop. Accepting the cost of 16 is a mechanism to break these loops.

Conclusion

The video provides a practical guide to constructing final forwarding tables using RIP and explains the conditions under which routers update their routing information. Understanding these concepts is crucial for designing and troubleshooting networks that use RIP. The emphasis on the cost of 16 and its role in preventing routing loops highlights a critical aspect of RIP's operation.