Section 4.3: Basic Switching Functions

In order to forward a packet that has arrived at a router interface, the router must perform a switching function. This switching function has four steps:

A packet transiting the router will be accepted into the router if the frame header contains the MAC address of one of the router's NIC cards. If properly addressed, the frame and its content will be buffered occurs in memory pending further processing.
The switching process checks the destination logical network portion of the packet header against the network/subnetwork entries in the routing table. If the search is successful, the switching process associates the destination network with a next-hop logical device and an outbound interface.
Once the next-hop logical device address is known, a lookup is performed to locate a physical address for the next device in the relay chain. The lookup is performed in an Address Resolution Protocol (ARP) table for LAN interfaces or a map table for WAN interfaces.
Once the physical address of the next-hop device is known, the frame header is overwritten, and the frame is then moved to the outbound interface for transmission onto the media. As the frame is placed on the media, the outbound interface adds the CRC character and ending delimiters to the frame. These characters will need to be validated at the arriving interface on the next-hop relay device.