Link State Algorithms

A link state algorithm advertises the link state and link metric for each of the connected links and directly connected routers to every router in the network.

OSPF and IS-IS are two link state routing protocols commonly used in enterprise and service provider networks.

Advertisements sent by OSPF are known as link-state advertisements, or LSAs. Advertisements sent by IS-IS are known as link-state packets or LSPs.

When a router receives an advertisement from a neighbour, the information is stored in a local database called the link state database. The link state information is passed on to each of the neighbouring routers of the local router in the same way that is was received – flooding through the network. The end result of this information flood is all routers in the network will have a synchronised and identical map of the network topology.

With the complete network topology, every router can run the Dijkstra shortest path first (SPF) algorithm calculation to determine the best shortest loop-free path. The routing table can be populated with this information.

In comparison to distance vector protocols, a link state protocol can consume much more CPU cycles and memory due to having a complete map of the network. The detailed map of the network works in the networks favour however with a reduced risk of routing loops and better network path decisions based on other factors rather than ‘hop count’.

Link state protocols are easily equipped with additional abilities, such as opaque LSAs for OSPF and TVLs (type/length/value) for IS-IS that allow features commonly used by service providers such as MPLS traffic engineering.



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