1.The Essential Core Switch layer 3 of the Backbone Network
No other words are better than "the mainstay" to describe the role of the core 3-layer switch among plenty of network devices. Among campus and metropolitan education networks, there are 3-layer switches everywhere in backbone networks, the backbone of metropolitan networks and their convergence layers. In particular, the core backbone network must adopt the 3- layer switches. Otherwise, a sub-net will be crowed with the entire network of thousands of computers. Therefore, neither security ensured nor broadcast domain divided to isolate the storm.
Although a traditional router is able to isolate the broadcast storm, it is inferior in its performance while that of the 3-layer switch is excellent. Equipped with both the three-layer’s routing and two-layer’s speed, the 3-layer switch employs the Layer 2 to switch based on MAC address and layer 3 to transmit traffic flows based on layer 3 address. Except for the necessary routing depending processes, most of the data transmission is processed by layer 2, which improves the efficiency of the packet switching.
The core 3-layer switch realizes the IP routing by the hardware switch mechanism. Its optimized routing software improves the efficiency as a remedy for that of the traditional one. Therefore, the 3-layer switch is combined with "router functions and switch performances ".
2.The Essential 3-layer Switch of the Sub-network
Aroused by excessive number of computers broadcasting on the same network (usually around 200, the number depends on the communication protocol) , the broadcast storm is likely to depress network transmission. As a way out, the network can be further divided into several virtual networks (VLAN) , which gives birth to a task that communication between VLANs must be done via routers. But the traditional router can hardly be competent for the communication tasks between VLANs for its routing ability is too weak to bear the network traffic.