Computer Networks · TEE · papers
Q3(b) · OSPF and link types
2024-12-06 · Final 2024-25 / Re-exam 2023-24
Actual paper; 100 marks attempted. Current-syllabus alignment is stated per question.
Q3(b). Explain in detail OSPF protocol. Summarise the different types of links used in OSPF routing protocol. [10 marks]
Answer
Definition. Open Shortest Path First (OSPF) is an interior link-state routing protocol that selects routes using shortest-path calculations and additive link costs.
Simple meaning. OSPF = link-state map + Dijkstra + summed costs.
Understand it step by step
- Routers discover and maintain neighbour relationships using Hello packets and exchange link-state information.
- Routers calculate shortest paths from their own position in the topology. A path cost is the sum of its outgoing-link costs, not necessarily the fewest hops.
- The network can be divided into areas. In normal OSPF hierarchy, Area 0 is the backbone used to interconnect other areas via area border routers.
- OSPFv2 is the IPv4 protocol in the supplied paper; it is carried directly in IP protocol number 89, not TCP/UDP port 89.
- A common vendor default derives interface cost from reference bandwidth/interface bandwidth, but actual configured costs and reference bandwidth matter. Use the given costs in an exam graph.
OSPF exchanges and maintains link-state information, builds a topology database and runs Dijkstra’s shortest-path computation rooted at the local router. It chooses a path with the lowest sum of link costs and installs the corresponding next hop. Equal-cost paths may be supported.
Yaad rakho: OSPF cost add karo; sirf hops nahi.
Link types represented in an OSPF topology
| Type | Meaning | Diagram |
|---|---|---|
| Point-to-point | Direct connection between two routers | R1 — R2 |
| Transit network | Multiaccess network through which routers communicate; a designated router helps represent it | R1 — LAN — R2 (also R3) |
| Stub network | Network reachable through one router, with no onward transit via another router | R1 — end-host LAN |
| Virtual link | Logical OSPF backbone connection through a transit area | Area 0 — R1 ⋯ transit area ⋯ R2 — Area 0 |
Do not confuse these topology/link descriptions with the five OSPF packet types. Hello discovers/maintains neighbours; database description, link-state request, link-state update and acknowledgement synchronize link-state information.