# Q3(b) · Count to infinity

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2024-ai-final/q3b

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2024-ai-final--q3b/

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

1. Before failure, B reaches a destination through A. After A loses the route, B may still advertise an old finite distance.
2. 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.
3. **Split horizon:** do not advertise a learned route back on the interface from which it was learned.
4. **Poison reverse:** advertise that reverse direction with an unreachable metric. **Route poisoning:** mark a failed destination unreachable. Triggered updates spread changes sooner.
5. These methods reduce common failure modes but do not constitute a universal proof that every multi-router loop is impossible.

#### Practical example

In RIP, a poisoned route is sent with metric 16. Receiving routers treat it as unreachable rather than a new usable longer route.

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.