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Link-state routing protocol features

Link-state routing protocol features 
6.2.6
The other basic algorithm that is used for routing is the link-state algorithm. This page will explain how the link-state algorithm works.
The link-state algorithm is also known as Dijkstra's algorithm or as the shortest path first (SPF) algorithm. The link-state routing algorithm maintains a complex database of topology information. The distance vector algorithm has nonspecific information about distant networks and no knowledge of distant routers. The link-state routing algorithm maintains full knowledge of distant routers and how they interconnect.
Link-state routing uses the following features:
  • Link-state advertisement (LSA) - a small packet of routing information that is sent between routers
  • Topological database - a collection of information gathered from LSAs
  • SPF algorithm - a calculation performed on the database that results in the SPF tree
  • Routing table - a list of the known paths and interfaces

Network discovery processes for link state routing
When routers exchange LSAs, they begin with directly connected networks for which they have information. Each router constructs a topological database that consists of all the exchanged LSAs.
The SPF algorithm computes network reachability. The router constructs this logical topology as a tree, with itself as the root. This topology consists of all possible paths to each network in the link-state protocol internetwork. The router then uses SPF to sort these paths. The router lists the best paths and the interfaces to these destination networks in the routing table. It also maintains other databases of topology elements and status details.
The first router that learns of a link-state topology change forwards the information so that all other routers can use it for updates. Common routing information is sent to all routers in the internetwork. To achieve convergence, each router learns about its neighbor routers. This includes the name of each neighbor router, the interface status, and the cost of the link to the neighbor. The router constructs an LSA packet that lists this information along with new neighbors, changes in link costs, and links that are no longer valid. The LSA packet is then sent out so that all other routers receive it.
When a router receives an LSA, it updates the routing table with the most recent information. The accumulated data is used to create a map of the internetwork and the SPF algorithm is used to calculate the shortest path to other networks. Each time an LSA packet causes a change to the link-state database, SPF recalculates the best paths and updates the routing table.
There are three main concerns related to link-state protocols:
  • Processor overhead
  • Memory requirements
  • Bandwidth consumption
Routers that use link-state protocols require more memory and process more data than routers that use distance vector routing protocols. Link-state routers need enough memory to hold all of the information from the various databases, the topology tree, and the routing table. Initial link-state packet flooding consumes bandwidth. In the initial discovery process, all routers that use link-state routing protocols send LSA packets to all other routers. This action floods the internetwork and temporarily reduces the bandwidth available for routed traffic that carries user data. After this initial flooding, link-state routing protocols generally require minimal bandwidth to send infrequent or event-triggered LSA packets that reflect topology changes.
This page concludes this lesson. The next lesson will provide an overview of routing protocols. The first page explains how a router performs path determination.

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