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CCIE EI How to understand the difference between Layer 3 switching and router

ccie enterprise infrastructure design

How to understand the difference between Layer 3 switching and router

I have studied network technology for a long time, but what is the difference between a layer 3 switch and a router? Many people will still be confused. So what is the difference between them, we need to take a closer look.

The simplest working principle of a Layer 2 switch is to forward data according to the MAC address table. There are four basic functions: learning, forwarding, broadcasting and updating.

When receiving a data frame, the switch will store the mapping relationship between the source MAC address of the data frame and the corresponding port number in the MAC address table for subsequent data forwarding. However, in this line of data forwarding, the target MAC address is used, and the MAC address table is queried. If the MAC address table has a corresponding mapping relationship, the data frame is forwarded unicast. If there is no corresponding mapping relationship, the general MAC address is used. flood. Another important feature is that if an entry is not used in the MAC address table for more than 300 seconds, the corresponding mapping relationship is deleted, that is, the update operation of the switch.

The router forwards data according to the routing table. If there is no corresponding entry in the routing table, the router will directly discard the data packet. A Layer 3 switch is a switch that has some router functions and works at the network layer. The most important purpose is to speed up data exchange within a large local area network. The routing function also serves this purpose, enabling one-time routing and multiple forwarding.

Layer 3 switches and routers also have routing functions, but this is just a function, just like many network devices now have the functions of multiple traditional network devices. For example, a router not only has routing functions, but also functions as a switch port and a hardware firewall, but in fact it is not a switch or a firewall. Its main function is routing, and the other is just its new additional capabilities. The reason is that we can pay a smaller price and have more complete functions. The layer 3 switch is still a switch, but only a switch with basic routing functions. It is still responsible for data exchange, and the interfaces are as many as before. But the router only has the main function of routing and forwarding, but not the function of a switch.

From the forwarding level, there is a big difference between routers and Layer 3 switches in data forwarding operations. Routers are generally forwarded based on software plus hardware, while Layer 3 switches perform data forwarding through hardware. After a layer 3 switch routes a data flow, it will generate a mapping table of MAC addresses and IP addresses. When the same data flow passes again, it will directly pass through the layer 2 instead of routing again according to this table. Reduce network delay and improve the efficiency of data packet forwarding. The forwarding of the router uses the longest matching method, which is complicated to implement, usually implemented by software, and the forwarding efficiency is low.

In terms of overall performance, the performance of Layer 3 switches is much better than that of routers, and it is very suitable for LANs with frequent data exchange. Although the router has powerful routing functions, its data packet forwarding efficiency is lower than that of Layer 3 switches, and it is more suitable for the interconnection of different types of networks where data exchange is not very frequent.

The routing function of the Layer 3 switch is simpler than that of a router, because what it does is mainly a simple LAN connection. Because of this, the routing function of Layer 3 switches is usually relatively simple, and the routing path is far less complicated than routers. The main function it uses in LAN is to provide fast data exchange function to meet the application characteristics of frequent data exchange and large flow of LAN. A router is different. It was originally designed to satisfy multiple types of network connections. Although it can be used for connections between LANs, its routing function is more reflected in the interconnection between different types of networks, such as multiple networks. Protocols, different network types, etc. It is essential to solve the connection of various complex routing paths network, so the routing function is very powerful. Its advantage lies in selecting the best route, load sharing, link backup and routing information exchange with other networks, etc. Router has the functions.

There is still a very substantial difference between Layer 3 switches and routers. Layer 3 switches cannot completely replace routers. The rich interface types, good traffic service level control, and powerful routing capabilities of routers are still the weak links of layer 3 switches. In summary, if multiple subnets are connected in a local area network, it is best to choose a layer 3 switch, especially in an environment where data exchange between different subnets is frequent. On the one hand, it can ensure the communication performance requirements, on the other hand, it also saves the investment of purchasing the layer 2 switch separately. It is best to determine according to the actual needs of your network



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