Discuss the count-to-infinity problem in distance vector routing algorithm with example.

Count-to-Infinity Problem in Distance Vector Routing: A Detailed Explanation

What is Distance Vector Routing?

• Routing protocol based on exchanging distance vectors between neighboring routers.
• Each router maintains a table of distances to all other networks.
• Routers share their distance vectors with directly connected neighbors.

Understanding the Count-to-Infinity Problem

• Occurs in distance-vector routing protocols like RIP (Routing Information Protocol).
• A routing loop creates a situation where the distance to a destination increases infinitely.
• This is because routers continuously update their routing tables with increasingly larger distances.

Example Scenario: Three Routers (A, B, C)

• Router A connected to B and C.
• Router B connected to A and C.
• Initially, A knows the distance to C through B is 2.
• B goes down, A updates distance to C as infinite.

Propagation of Incorrect Information

• A tells C its distance to B is infinite.
• C tells A its distance to B is infinite + 1 (cost to reach B).
• A updates its distance to B to infinite + 2.
• This cycle repeats until the metric value overflows.

Solutions to the Count-to-Infinity Problem

• Using split horizon with poisoned reverse: A router does not send the distance to a destination back to the interface it learned the information from.
• Using hold-down timers: Prevents unstable routes from repeatedly propagating.
• Using path vector protocols: Protocols like OSPF or BGP that avoid this problem altogether by employing different routing mechanisms.