Networking —

Link State Routing

Link state routing works quite differently from distance vector-based routing. Rather than each router telling each other connected router about the routes it is aware of, routers in a link state environment send out special packets, called link state advertisements (LSA), which contain information only about that router. These LSAs are forwarded to all the routers on the network, which enables them to build a map of the entire network. The advertisements are sent when the router is first brought onto the network and when a change in the topology is detected.

Of the two (distance vector and link state), distance vector routing is better suited to small networks and link state routing to larger ones. Link state protocols do not suffer from the constant updates and limited hop count, and they are also quicker to correct themselves (to converge) when the network topology changes.

On TCP/IP networks, the most commonly used link state routing protocol is the Open Shortest Path First (OSPF). On IPX networks, the NetWare Link State Protocol (NLSP) is used. Table 1 summarizes the distance vector and link state protocols used with each network protocol.

Table 1 Routing Protocols

Network Protocol

Distance Vector

Link State

TCP/IP

RIP

OSPF

IPX/SPX

RIP*

NLSP