# Q3(b) · RIP and timers

Subject: Computer Networks

Exam: TEE

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

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

Actual paper; 100 marks attempted. Current-syllabus alignment is stated per question.

### Q3(b). Explain in detail the working of Routing Information Protocol (RIP). List and briefly explain the timers used in RIP. [10 marks]

### Answer

**Definition.** Routing Information Protocol (RIP) is an interior distance-vector routing protocol that uses a bounded metric, commonly hop count, to select routes.

**Simple meaning.** RIP mein hops count; 15 tak reachable, 16 means unreachable.

## Understand it step by step

1. RIPv2 uses distance-vector updates and carries subnet-mask information, allowing classless routes.
2. The usual hop-count metric admits costs 1–15 for reachable routes; 16 denotes infinity/unreachable. A received advertisement is incremented by the local link cost and capped at 16.
3. RIPv2 normally sends periodic updates about every 30 seconds, with implementation/timer behavior and triggered updates affecting the exact timing. RIPv2 uses UDP port 520 and can send updates to multicast 224.0.0.9.
4. Hop count does not directly measure bandwidth or delay, so a low-hop route is not always the fastest link path. The 15-hop limit restricts scale.

#### Practical example

If a neighbour advertises a destination at 15 hops and your added cost is 1, the resulting 16 means unreachable, not a valid 16-hop route.

RIP uses distance-vector routing and commonly hop count as its metric. Reachable metrics are at most 15; 16 represents infinity. Its main limitations include limited diameter, a coarse metric and relatively slow convergence with possible temporary loops.

**Yaad rakho:** RIP: neighbour distances; 16 is infinity.

### RIPv2 timer roles

| Timer | Usual RFC value | Action |
| --- | --- | --- |
| Periodic update | Approximately 30 seconds, with randomized variation | Send routing information to neighbours |
| Route timeout | 180 seconds without a valid refresh | Declare the route invalid and set metric 16 |
| Garbage collection | 120 seconds after invalidation | Keep advertising the unreachable route before deleting it |

Some vendor courses additionally describe an invalid/hold-down/flush timer set (often 180/180/240 seconds). Those are implementation conventions and must not be mixed with the RFC timeout-plus-garbage sequence as if they were identical clocks.