Routing Protocols

Protocols

Routing Protocols

Network layer routing protocols provide the communication protocols and algorithms for determining routes to every destination and also for distributing the routing information throughout the network. Routing protocols are categorized as either interior (intradomain) or exterior (interdomain) routing protocols, with interior protocols designed to work only within a domain or autonomous system. An algorithm that is efficient at intradomain routing might not be well-suited for interdomain routing. The LAN subsystem of netWorks provides simulations for several interior routing protocols designed for broadcast networks.

All popular routing protocols are based on one of the following distributed algorithms:

Distance Vector
Link State

The first routing protocols employed distance vector routing. In this scheme each router keeps track of the distance to each of its neighbors and then transmits this vector to every neighbor. The routers then use the set of all distance vectors they receive to determine the shortest paths to each destination.

In link state algorithms, routers determine their neighbors (and the cost to each) by exchanging hello packets. They then construct link state packets containing a list of their neighbors (and associated costs) and flood these packets throughout the entire domain. Using the link state packets, the routers can construct a graph of the network and then calculate the "best" paths through it. Link state algorithms converge more quickly than distance vector algorithms, but this typically require more memory and CPU processing than distance vector algorithms.

The netWorks application provides simulations of two link state protocols:

Intermediate System - Intermediate System (IS-IS)
Open Shortest Path First (OSPF)

Both of these protocols use the notion of hierarchical routing to reduce the amount of link state information the routers need to exchange. In hierarchical routing, the network is subdivided into separate, logical units typically called areas, and routers share their link state information with other routers in their area. Some routers have additional responsibilities to forward packets between areas. These are considered to be at a higher level in the hierarchical routing scheme, and the routing protocol defines separate procedures for forwarding packets within an area versus between areas.

The IS-IS and OSPF protocols both elect a Designated Router (DR) on LANs with more than one router attached to them. The protocols typically use the router's priority and router ID in the DR selection process. The DR is responsible for collecting and distributing the link state information for all the routers on its LAN throughout its area. This helps reduce the protocol traffic on the network. In the OSPF routing protocol, a Backup Designated Router (BDR) is also elected to take over should the DR fail. The following sections provide a brief overview of the IS-IS and OSPF routing protocols.

IS-IS

OSPF

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