Link state

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A link-state routing protocol is a network protocol that is used by routers to build a complex database of topology information. With the help of this database , the packets are then forwarded in the network. Frequently occurring representatives of this type of protocol on the Internet are z. B. OSPF or IS-IS .


Link-state routing protocols work very differently than distance vector protocols like RIP or IGRP . Distance vector protocols usually only exchange information about the accessible networks with their direct neighbors and have no knowledge of the complete network topology. This contrasts with the more complex routing information that is exchanged between the routers in the case of link-state routing protocols.

Classless interdomain routing CIDR as well as subnet masks with variable length VLSM are supported by all common link state routing protocols.

Working method

If there are many changes in the routing table and the routing table needs to be updated frequently or regularly, it is recommended that you use a link-state routing protocol. Only the respective changes are exchanged between the routers. With link state routing, so-called LSA (link state announcements / advertisements) are announced by flooding when changes are made in the network . With OSPF and IS-IS this is done to a multicast address so that only interested network participants (OSPF or IS-IS router) process this information further. The entire topology of the network is generated in the topology database of each router on the basis of the LSAs received. Since the changes are propagated to the neighboring router in a connection-oriented manner, routing protocols with the LSA have good stability.

The link-state algorithm is the practical implementation of the Dijkstra algorithm from graph theory.

Features of link state routing

  • Works with the SPF algorithm (Shortest Path First) and the resulting SPF tree
  • Regular updates (link state updates) through flooding
  • Establishing the availability of neighbors using the Hello protocol
  • Fast reaction to network changes: the SPF algorithm uses the LSA information to recalculate the optimal paths and updates the routing table (local)
  • The routing table contains the path with costs and interfaces to every known network in order to determine the optimal path for the packets.

See also