Routing is a relay system by which packets are forwarded from one device to another. Each device in the
network as well as the network itself has a logical address so it can be identified and reached individually or
as part of a larger group of devices. For a router to act as an effective relay device, it must be able to
understand the logical topology of the network and to communicate with its neighboring devices. The router
understands several different logical addressing schemes and regularly exchanges topology information with
other devices in the network. The mechanism of learning and maintaining awareness of the network
topology is considered to be the routing function while the movement of traffic through the router is a
separate function and is considered to be the switching function. Routing devices must perform both a
routing and a switching function to be an effective relay device. A router receiving a packet from a host, the
router will need to make a routing decision based on the protocol in use; the existence of the destination
network address in its routing table; and the interface that is connected to the destination network. After the
decision has been made the router will switch the packet to the appropriate interface on the router to forward
it out. If the destination logical network does not exist in the routing table, routing devices will discard the
packet and to generate an Internet Control Message Protocol (ICMP) message to notify the sender of the
event.