For better or for worse, Layer 3 switching is implemented in many different ways by different vendors. Even in Cisco’s line of Catalyst switches, many different methods are used depending upon the hardware series and model. The different methods used by Catalyst switches will be looked at shortly. For now, let’s reexamine the process by which a traditional router forwards traffic.
Imagine a very simple network that consists of two hosts that are part of different VLANs on the same switch. Each VLAN is connected to a different port on a traditional router. Recall that even though the two hosts are connected to the same switch, routing is necessary in order for them to communicate, since they are members of different VLANs. Host A will create a packet listing itself as the source IP address, and Host B as the destination IP address. It will then frame the packet, listing itself as the source MAC address, and router interface E0 as the destination MAC address. Once complete, the frame is forwarded across the network to the router.
When the frame arrives at the router’s E0 interface, the CRC is calculated, the MAC framing is stripped away, and the packet is passed to the Network Layer. At this layer the router calculates the IP checksum, and then examines the routing table to determine where the packet should be forwarded next. After decrementing the packet’s TTL by 1, the router reframes the packet with its E1 interface as the source MAC address, and the MAC address of Host B as the destination. Once complete, the packet is forwarded back to the switch, and ultimately to Host B. All this work for just one routed packet!
Ultimately, every routed packet is forwarded in this manner on a traditional IP network. Even though many packets may ultimately need to be forwarded between the same two hosts, this process must still be completed. As you’ll see shortly, Layer 3 switching techniques go a long way towards reducing the amount of overhead associated with routing packets between networks.