LAYER-3 SWITCHING

A large community holds that the use of an underlying Frame Relay or ATM switching fabric is not efficient because it adds additional, superfluous logic and overhead to the system. This community is involved with the design of routers that operate directly on the IP address, and do not correlate the address to a label. This approach focuses on a multigigabit router (MGR), that uses Layer-3 switching techniques.

Figure 12-12 shows the overall architecture for the MGR. We will assume the packets enter the router from the left and exit to the right. The router contains multiple line cards (which support one or more interfaces), and forwarding engine cards, all connected to a switch. The arriving packet has its header removed and passed through the switch to the forwarding engine card, while the other part of the packet is stored on the incoming line card. The forwarding engine examines the header, determines the routing for the packet, updates the header, and forwards it back to the incoming line card, with associated forwarding information. Then, this line card appends the revised header to the other part of the packet, and sends the reconstituted packet to the appropriate outgoing line card.