Link-State Routing Protocols

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Link-State Routing Protocols Overview

Link-State Routing protocols are routing protocols whose algorithms calculate the best paths to networks differently than Distance Vector routing protocols. Whereas Distance Vector protocols know routes by measures of distance and vector(direction) as reported by neighboring routers, Link-State routing protocols calculate their network routes by building a complete topology of the entire network area and then calculating the best path from this topology or map of all the interconnected networks.

{loadposition adposition5}There are two link-state routing protocols, OSPF and IS-IS. The Cisco CCNA curriculum covers the Open Shortest Path First or OSPF link-state routing protocol, and the IS-IS routing protocol is part of the CCNP curriculum.

Link-State Characteristics

  • SPF algorithm – Link-State routing protocols are designed around Dijkstra’s Shortest Path First Algorithm (SPF) in which the shortest path from point A to point B is build around a metric of cost.
  • Cost metric – SPF algorithm finds the shortest path based on a metric network link costs. Each router measures the cost of its own directly connected networks or "links." Cost is a measure of the quality of a link based mostly on bandwidth.
  • Hello packets – Link-State routing protocols establish adjacencies with neighboring routers using hello packets.
  • Link State Packets (LSP) – Initial flooding of link-states to all routers in the network.
  • Topology or SPF Tree – Link-State routing protocols build and maintain a complete map or topology of the network area.   {loadposition adposition6}

Link-State Advantages

  • Faster Convergence – Unlike Distance Vector routing protocols which run algorithm calculations before sending updates, Link-State routing protocols send link-state updates to all routers in the network before running route calculations
  • Triggered Updates – Unlike Distance Vector routing protocols (except EIGRP) which send periodic updates at regular intervals, Link-State routing protocols send LSPs during router startup (flooding) and when a link changes states like going up or down. If their are no changes in the network the protocol only sends hello packets to maintain adjacencies.
  • Scalability – Link-State routing protocols support the ability to configure multiple routing "areas" which allows an administrator to segment a routing protocol processes to defined areas which supports the expansion and troubleshooting of much larger networks.

Link-State Disadvantages

  • Greater Processing Requirements – Link-State routing protocols typically demand greater processing power and memory resources from the router.
  • Greater Administrator Knowledge – Link-State routing protocols can demand advanced administrator knowledge to configure and troubleshoot the network area


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Author: Dan

Dan teaches computer networking and security classes at Central Oregon Community College.

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