Computer Networks · TEE · papers
Q3(b) · RIP and timers
2024-02-06 · 2022-23
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
- RIPv2 uses distance-vector updates and carries subnet-mask information, allowing classless routes.
- 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.
- 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.
- 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.
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.