Computer Networks · TEE · papers
Q3(b) · Count to infinity
2024-12-06 · AI-only Sem V variant
Exact historical variant. Current-syllabus questions solved; excluded topics and source limitations stated.
Q3(b). What is the count-to-infinity problem in distance-vector routing? [5 marks]
Answer
Definition. Count-to-infinity is a distance-vector failure in which routers repeatedly increase their estimates for an unreachable destination because they believe one another’s stale route advertisements.
Simple meaning. Dono neighbour ek dusre ko bol rahe: mere through jao—par actual route gaya.
Understand it step by step
- Before failure, B reaches a destination through A. After A loses the route, B may still advertise an old finite distance.
- Without sufficient protection, A may infer a new route through B even though B’s route depended on A. This creates a loop and rising metrics.
- Split horizon: do not advertise a learned route back on the interface from which it was learned.
- Poison reverse: advertise that reverse direction with an unreachable metric. Route poisoning: mark a failed destination unreachable. Triggered updates spread changes sooner.
- These methods reduce common failure modes but do not constitute a universal proof that every multi-router loop is impossible.
Split horizon suppresses advertising a route back in the direction from which it was learned. Poison reverse explicitly advertises that route back with an infinite/unreachable metric. Both reduce two-router distance-vector loops caused by stale information.
Yaad rakho: Split: wapas mat bolo. Poison reverse: wapas bolo, par unreachable.