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What is count-to-infinity problem in distance vector routing protocol? How does it happen? Explain with an example.

Count-to-Infinity Problem in Distance Vector Routing Protocol:

The count-to-infinity problem is a scenario that can occur in distance vector routing protocols like RIP (Routing Information Protocol) or RIPv2. In this problem, routers in a network incorrectly determine the shortest path to a destination, leading to routing loops and incorrect routing table entries.

How it Happens:

The count-to-infinity problem occurs due to slow convergence in distance vector routing protocols. When a link goes down, routers must update their routing tables to reflect this change. In distance vector protocols, routers exchange routing information with their neighbors, and each router makes routing decisions based on the information received from its neighbors.

However, if there is a change in the network topology (such as a link failure), routers may receive inconsistent information from their neighbors about the availability of routes. In some cases, a router may receive information that indicates a failed route but then receives updates from other routers indicating that the route is still available, albeit with a higher cost. This conflicting information can cause routers to incorrectly increase the metric (cost) of a route, leading to a situation where routers keep updating their routing tables with increasing costs, ultimately leading to a loop.

Example:

Consider a network where Router A, Router B, and Router C are connected in a triangle. Initially, Router A believes it has the shortest path to Router C with a cost of 1. When the link between Router A and Router C goes down, Router A informs Router B about the failure. Router B updates its routing table, marking the path to Router C as unreachable. However, Router C still believes it can reach Router A through Router B with a cost of 1.

Now, Router B informs Router C that the path to Router A is unavailable, so Router C increases the cost to reach Router A through Router B to 2. But Router B still believes it can reach Router A with a cost of 1 via Router C. This back-and-forth continues, with each router incrementing the cost to reach Router A, causing a count-to-infinity problem.

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