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Router Technologies

Router Technologies

Routers
Whereas switches and bridges operate at OSI Layer 2 (data link layer), routers primarily operate at OSI Layer 3 (network layer). Like bridging, the primary act of routing involves moving packets across a network from a source to a destination. The difference involves the information that is used to make the forwarding decisions. Routers make decisions based on network layer protocols such as Internet Protocol (IP) and Novell NetWare Internetwork Packet Exchange (IPX).

Routing gained popularity in the mid to late 1980s as a result of internetworks growing beyond the capability of bridges. Before this popularity, networks were relatively small and isolated, and bridges were able to handle the jobs of forwarding and segmentation. However as networks grew, routers facilitated larger scaling and more intelligent growth across wider physical distances. Although routers are more expensive and complex than bridges, routing is the core of the Internet today. (As a side note, Cisco as a company made its name through routing.)

Routing involves two processes: determining optimal routing paths through a network and forwarding packets along those paths. Routing algorithms make the optimal path determination. As they determine routes, tables on the router store the information.

Routing algorithms fill routing tables with various types of information. The primary piece of information relevant to routing is the next hop. Next-hop associations tell a router that it can reach a particular destination by sending a packet to a particular router representing the next hop on the way to its final destination. When a router receives a packet, it attempts to associate the destination network address in the packet to an appropriate next hop in its routing table. In addition to next-hop associations, routers store other pertinent information in routing tables. For multiple paths to a destination, a routing table might contain information that allows the router to determine the desirability of one path over another.

Routers communicate with each other and maintain their routing tables through the exchange of messages over the network. Routing updates are one particular type of message. A routing update contains all or part of another router’s routing table and allows each router to build a detailed picture of the overall network topology.

Once a router determines an optimal path for a packet, it must forward the packet toward the destination. The process of a router moving a packet from its received port to the outgoing destination port is called switching. Although the process of switching a packet on a router is similar to that of a Layer 2 switch, the decision criteria and the actual handling of the packet are different.

When a computer determines that it must send a packet to another host, it places the network address of the final destination host in the packet. However, it places the Layer 2 physical (Media Access Control [MAC]) address of the nearest router in the packet. When the router receives the packet, it first determines whether it knows how to reach the packet’s stated destination network. If the destination is not known, the router typically drops the packet. If the destination is known, the router changes the destination physical address in the packet to contain that of the next hop. The router then transmits the packet out the destination interface.

The next hop can be either the final destination or another router. Each router in the process performs the same operation. As the packet moves through the network, each router modifies the physical address stored in the packet but leaves the network address untouched (because it determines the final destination).




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