Link State Routing Facts

Link State Routing Facts

Keep in mind the following information about the link-state method.

• Routers broadcast Link-State Packets (LSPs) to all routers (this process is known as flooding).
• Routers send information about only their own links.
• Link-state protocols send hello packets to discover new neighbors.
• LSPs are sent at regular intervals and when any of the following conditions occur.
  • There is a new neighbor.
  • A neighbor has gone down.
  • The cost to a neighbor has changed.
• Neighboring routers exchange Link-state Advertisements (LSAs) to construct a topological database.
• The Shortest Path First (SPF) algorithm is applied to the topological database to create an SPF tree from which a table of routing paths and associated ports is built.
• Routers use LSPs to build their tables and calculate the best route.
• Routers use the SPF algorithm to select the shortest route.
• Network administrators have greater flexibility in setting the metrics used to calculate routes.

The link-state method has the following advantages over the distance vector method.

• Less convergence time (because updates are forwarded immediately)
• Not susceptible to routing loops
• Less susceptible to erroneous information (because only firsthand information is broadcast)
• Bandwidth requirements negligible for a typical LAN environment

Although more stable than the distance vector method, the link-state method has the following problems:

• The link-state algorithm requires greater CPU and memory capability to calculate the network topology and select the route because the algorithm re-creates the exact topology of the network for route computation.
• It generates a high amount of traffic when LSPs are initially flooded through the network or when the topology changes. However, after the initial configuration occurs, the traffic from the link-state method is smaller than that from the distance vector method.
• It is possible for LSPs to get delayed or lost, resulting in an inconsistent view of the network. This is particularly a problem for larger networks, if parts of the network come on line at different times, or if the bandwidth between links varies (i.e. LSPs travel faster through parts of the network than through others).

In particular, the last problem is of greatest concern. The following solutions are often implemented to overcome some of the effects of inconsistent LSP information.

• Slowing the LSP update rate keeps information more consistent.
• Routers can be grouped into areas. Routers share information within the area, and routers on area borders share information between areas. (Areas logically subdivide an Autonomous System (AS), a collection of areas under common administration.)
• One router in each area is designated as the authoritative source of routing information (called a designated router). Each area router receives updates from the designated router.
• LSPs can be identified with a time stamp, sequence or ID number, or aging timer to ensure proper synchronization.