In the distribution layer of an enterprise network, routing is required. Without the routing process, packets cannot leave the local network.

Routers play a critical role in networking by interconnecting multiple sites within an enterprise network, providing redundant paths, and connecting ISPs on the Internet. Routers can also act as a translator between different media types and protocols. For example, a router can accept packets from an Ethernet network and re-encapsulate them for transport over a Serial network.

Routers use the network portion of the destination IP address to route packets to the proper destination. They select an alternate path if a link goes down or traffic is congested. All hosts on a local network specify the IP address of the local router interface in their IP configuration. This router interface is the default gateway.

Routers also serve other beneficial functions:

Click each highlighted area in the figure for more information on the functions of routers.

With the enterprise and the ISP, the ability to route efficiently and recover from network link failures is critical to delivering packets to their destination.