Figure Routers in an Internet

Routing Concepts  Least Cost Routing: The decision of the least-cost routing is based on efficiency: which of the available pathways is the cheapest or, in networking terminology, the shortest.  The term shortest, in this context, can mean either of two things depending on the protocols. In some cases, shortest mean the route requiring the smallest number of relays or hops.  In other cases, shortest means fastest, cheapest, most reliable, most secure, or best of any other quality that can make one particular link more attractive then another. Usually, shortest means a combination of all of these.

Adoptive versus Nonadaptive  In adaptive Routing, by which a router may select a new route for each packet in response to changes in condition and topology of the networks.  In non adaptive routing protocols, once a pathway to a destination has been selected, the router sends all packets for that destination along that one route. The routing decisions are not made based on the condition or topology of the networks.

Figure A Gateway in the OSI Model

Gateways  Gateways potentially operate in all seven layers of the OSI model.  Gateway is a protocol converter  The Gateway must adjust the data rate,size and format as well.

Figure A Gateway IBM network Novell network

Figure Single-Protocol versus Multiprotocol Router

Figure Brouter

Switches  The switch normally has a buffer for each link (network) to which it is connected.  When it receive a packet, it store the packet in the buffer of the receiving link and checks the address to find the outgoing link.  If outgoing link is free the switch sends the frame to that particular link.

Figure Switch

Routing Algorithms  Two common methods are used to calculate the shortest path b/w two routers. 1.Distance vector routing 2.Link state routing.  In distance vector routing, each router periodically shares its knowledge about the entire network with its neighbors. In distance routing, the cost is based on hop count.

Figure Example of an Internet

Figure The Concept of Distance Vector Routing