Link State Routing
Complete topology awareness using Dijkstra's shortest path algorithm
Key Protocols
OSPF, IS-IS, and other implementations
Convergence
Fast convergence with complete topology knowledge
Scalability
Hierarchical areas for large networks
Link State Topics
Link State Algorithm Details
Link State Algorithm Steps
Network Discovery
Each router discovers its neighbors and link costs
Routers send HELLO packets to identify neighbors and measure link costs
LSA Generation
Create Link State Advertisements (LSAs)
Each router creates LSA containing its neighbors and link costs
LSA Flooding
Flood LSAs throughout the network
LSAs are reliably flooded to all routers using sequence numbers
Topology Database
Build complete network topology
Each router builds identical link-state database of entire network
SPT Calculation
Run Dijkstra's algorithm
Calculate shortest path tree (SPT) from this router to all destinations
Routing Table
Install routes in forwarding table
Extract next-hop information from SPT for routing table
Link State vs Distance Vector
| Aspect | Link State | Distance Vector |
|---|---|---|
| Information Shared | Complete topology (LSAs) | Distance vectors only |
| Convergence Speed | Fast (immediate flooding) | Slow (iterative updates) |
| Routing Loops | No loops (complete topology) | Possible during convergence |
| Memory Usage | High (topology database) | Low (distance vectors) |
| CPU Usage | High (Dijkstra calculation) | Low (simple computation) |
| Scalability | Good (hierarchical areas) | Limited (hop count) |
📝 Exam Preparation Tips
Key Topics to Remember
- • Link state algorithm uses Dijkstra's SPF
- • LSAs flooded throughout area/domain
- • All routers have identical topology database
- • OSPF uses areas for scalability
- • Different LSA types for different information
Common Exam Questions
- • "Explain link state algorithm steps" (8 marks)
- • "Compare OSPF vs RIP" (10 marks)
- • "Describe OSPF area concept" (6 marks)
- • "Types of LSAs and their purposes" (8 marks)
- • "Link state vs distance vector routing" (10 marks)