As a packet travels through an internetwork to its final destination, the Layer 2 frame headers and trailers are removed and replaced at every Layer 3 device. This is because Layer 2 data units, frames, are for local addressing. Layer 3 data units, packets, are for end-to-end addressing.

Layer 2 Ethernet frames are designed to operate within a broadcast domain using the MAC address that is burned into the physical device. Other Layer 2 frame types include Point-to-Point Protocol (PPP) serial links and Frame Relay connections, which use different Layer 2 addressing schemes. Regardless of the type of Layer 2 addressing used, frames are designed to operate within a Layer 2 broadcast domain, as the data crosses a Layer 3 device the Layer 2 information changes.

As a frame is received at a router interface, the destination MAC address is extracted. The address is checked to see if the frame is directly addressed to the router interface, or if it is a broadcast. In either of these two cases, the frame is accepted. Otherwise, the frame is discarded since it is destined for another device on the collision domain. The accepted frame has the Cyclic Redundancy Check (CRC) information extracted from the frame trailer, and calculated to verify that the frame data is without error. If the check fails, the frame is discarded. If the check is valid, the frame header and trailer are removed and the packet is passed up to Layer 3. The packet is then checked to see if it is actually destined for the router, or if it is to be routed to another device in the internetwork. If the destination IP address matches one of the router ports, the Layer 3 header is removed and the data is passed up to the Layer 4. If the packet is to be routed, the destination IP address will be compared to the routing table. If a match is found or there is a default route, the packet will be sent to the interface specified in the matched routing table statement. When the packet is switched to the outgoing interface, a new CRC value is added as a frame trailer, and the proper frame header is added to the packet. The frame is then transmitted to the next broadcast domain on its trip to the final destination.

Interactive Media Activity Drag and Drop: Packet Propagation Flowchart After completing this activity, the student will be able to better understand the detailed packet propagation process that goes on within a router.