Basic exchange: A comprehensive description of layer-3 Switch principles

The layer-3 Switch has become one of the most important devices in the network. Here we analyze the technical principles of the layer-3 switch in the exchange technology, a device with the layer-3 switch function is a layer-2 switch with the layer-3 routing function, but it is an organic combination of the two, it is not simply to overlay the hardware and software of the router device on the LAN switch.

From the hardware implementation point of view, at present, the interface modules of the second layer switch exchange data through the High-Speed Backplane/bus (the speed can reach dozens of Gbit/s, in layer-3 vswitches, layer-3 router-related Routing hardware modules are also inserted on the high-speed backplane/bus, in this way, the routing module can exchange data with other modules to be routed at a high speed, this breaks through the speed limit of traditional external router interfaces (10 Mbit/s --- 100 Mbit/s). In terms of software, layer-3 switches also have major initiatives, it defines the traditional software-based router software. The practice is:

1. For data packet forwarding, such as IP/IPX packet forwarding, these regular processes are implemented through hardware at high speed.

2. For layer-3 software, such as route information update, route table maintenance, route calculation, and route determination, Use optimized and efficient software. Assume that two sites using the IP protocol communicate with each other through the layer-3 Switch. When sending Site A, the IP address of the destination site is known, however, you do not know the MAC address required to send messages on the LAN. IP Address Resolution (ARP) is used to determine the MAC address of the target site. The sending site compares its IP address with the IP address of the destination site and extracts the network address using the subnet mask configured in the software to determine whether the destination site is in the same subnet as itself.

If the destination site B and the sender Station A are in the same subnet, A broadcasts an ARP request, and B returns its MAC address, after obtaining the MAC address of the target site B, A caches the address and uses the MAC address to package and forward data. The second layer switch module looks for the MAC address table to determine whether to send data packets to the target port. If two sites are not in the same subnet, if Station A needs to communicate with Station C, station A must send an ARP (Address Resolution) packet to the "Default Gateway, the IP address of the "Default Gateway" has been set in the system software. This IP address actually corresponds to the layer-3 Switch module of the layer-3 switch.

Therefore, when station A broadcasts an ARP request to the IP address of the "Default Gateway", if the layer-3 Switch module obtains the MAC address of the target Station B during the previous communication, then, send A reply to the MAC address of B to the sender. Otherwise, the layer-3 Switch module broadcasts an ARP request to the destination based on the route information, after receiving the ARP request, the target Station C returns its MAC address to the layer-3 Switch module. The layer-3 Switch module saves the address and sends it back to the sending station. Later, when packet forwarding between A and C is performed, the MAC address packet of the destination site will be used, and the data forwarding process will be handed over to the second layer for exchange, so that information can be exchanged at A high speed.

The layer-3 Switch has the following outstanding features: the combination of organic hardware accelerates data exchange; the optimized routing software improves the routing efficiency; in addition to the necessary routing decision processes, most data forwarding processes are processed by Layer 2 switching. When multiple subnets are connected, they are only logically connected to the Layer 3 Switching Module. Unlike traditional external routers, ports need to be added, this protects users' investment.