Distance Vector vs Link State Routing

Basis	Distance Vector	Link State
Algorithm	Bellman-Ford	Dijkstra (shortest path first)
Information shared	Whole routing table (vector of distances)	Link-state of own links (LSA)
Shared with	Neighbours only	All routers (flooding)
Knowledge of topology	Only distance and next hop	Complete map of the network
Updates	Periodic (e.g. 30 s in RIP)	Event-triggered, on topology change
Convergence	Slow; count-to-infinity problem	Fast
Resource use	Low CPU and memory	High CPU and memory
Examples	RIP, IGRP	OSPF, IS-IS

Working of Distance Vector

1. Each router starts knowing only the distance (cost) to its directly connected neighbours.
2. Periodically it sends its distance vector (destination, cost) to its neighbours.
3. On receiving a vector, a router updates its table with the Bellman-Ford equation: $d_x(y) = \min_v ( c(x,v) + d_v(y) )$.
4. This repeats until tables become stable (convergence). Problems: slow convergence and count-to-infinity, reduced by split horizon and poison reverse.

Working of Link State

1. Each router discovers its neighbours (Hello packets) and measures link costs.
2. It builds a Link State Advertisement (LSA) listing its links and costs.
3. The LSA is flooded to all routers, so every router has the same link-state database (full topology).
4. Each router runs Dijkstra's algorithm to build a shortest-path tree with itself as root and fills its routing table from it.