What does a layer 3 network do? What are the functions of each layer in the 3 layer hierarchical net

The essential difference between Layer 2 and Layer 3 is whether multiple int vlanif interfaces can be configured, which can be a Layer 3 switch, and those that can only configure one int vlanif are usually called managed switches, and those that cannot be configured with int vlanif are called ordinary Layer 2 switches. The details are as follows:

1. Layer 2 and layer 3 are classified according to the logical topology, which does not mean that the data link layer and network layer in the ISO seven-layer model, but refers to the core layer, aggregation layer and access layer, which are deployed in the three-layer network structure, and the second-layer network structure does not have an aggregation layer. 2. The Layer 2 network can communicate only through MAC addressing, but only within the same conflict domain; A Layer 3 network needs to communicate across network segments through IP routes, which can span multiple collision domains.

3. The networking capability of the Layer 2 network is very limited, and it is generally only a small local area network; Layer 3 networks can be used to form large networks.

4. The Layer 2 network is basically a security domain, that is to say, in the same Layer 2 network, the security of the terminal is basically the same from the network, unless there are other special security measures; A Layer 3 network can be divided into multiple security domains that are relatively independent.

5. Many technologies are relatively much used in the Layer 2 LAN, such as DHCP, shared connections provided by Windows, etc., if you need to use them on the Layer 3 network, you need to consider the support of other devices (such as through DHCP relay**, etc.) or through other ways to achieve.

6. In the actual application process, the typical practice is: the interconnection of various subnets in the same LAN and the routing between VLANs in the LAN are replaced by three-layer switches, and only when the network and the public network interconnection want to achieve cross-regional network access, only through the professional router.

1. Core layer: the high-speed switching backbone of the network.

2. Aggregation layer: provides policy-based connectivity.

3. Access layer: connect the workstation to the network.

The three-layer network architecture adopts a hierarchical model design, that is, the complex network design is divided into several levels, each level focuses on some specific functions, so that a complex big problem can become many simple small problems.

The OSI model structure is divided into seven layers, from the lowest layer to the highest layer, which are the physical layer, the data link layer, the network 3, and the network layer The network layer is a complex layer, which is responsible for providing connectivity and path selection. If you want to use it.

The three layers of the network are the core layer, aggregation layer, and access layer.

The core layer is the aggregation and processing point for all traffic, and it enables the high-speed exchange of packets.

The main function of the aggregation layer is to aggregate network traffic and shield the impact of changes in the access layer on the core layer. (The convergence layer plays the role of connecting the previous and the next.) ）

The access layer provides end users with the ability to access the network.

In the selection of enterprise network structure, there are two options: Layer 2 network and Layer 3 network structure.

Here, the second and third layers are classified according to the seepage logic topology, which does not mean that the data link layer and network layer in the ISO seven-layer model, but refers to the core layer, aggregation layer and access layer, which are all deployed in the three-layer network structure, and the second-layer network structure does not have an aggregation layer.

Only the core layer and the access layer of the layer two network structure mode is easy to operate, the switch according to the MAC address table for the packet, if there is a **, if not, then flood, that is, the packet broadcast is sent to all ports, if the destination terminal is late to give a response, then the switch can add the MAC address to the address table, this is the process of the switch to establish the MAC address, but so frequently broadcast the packets of the unknown MAC destination, The network storm formed in the large-scale network architecture is very large, which also greatly limits the expansion of the scale of the layer 2 network, so the networking capacity of the layer 2 network is very limited, so it is generally only used to build a small local area network.

Unlike the Layer 2 network structure, the Layer 3 network structure can form large networks.

Therefore, in the entire three-layer network structure, the equipment requirements of the core layer are the highest, and it must be equipped with high-performance data redundancy transfer equipment and load-balancing equipment to prevent overload, so as to reduce the amount of data that each core layer switch needs to carry. (High-speed switching backbone of the network).

The aggregation layer is the core layer that connects the network and the application layer of each access, and plays the role of "media transmission" between the two layers. The aggregation layer should have the following functions: implement security functions (VLAN division and ACL configuration), overall workgroup access function, and virtual network filtering function.

Therefore, Layer 3 switches should be used for aggregation layer devices. (Provides policy-based connectivity).

The access layer is mainly object-oriented to end customers and provides access functions for end customers. (Connect the workstation to the network).

Layer 2 networks can communicate only through MAC addressing, but only within the same collision domain; Layer 3 networks need to communicate across network segments through IP routes, which can span multiple conflict domains.

The Layer 3 switch can replace the router to a certain extent, but it should be clearly realized that the most important purpose of the Layer 3 switch is to speed up the data exchange within the large LAN, and the routing function is mostly around this purpose, so his routing function is not as strong as the professional router of the same grade, and there are still many deficiencies in security, protocol support, etc., and it cannot completely replace the router.

In the process of practical application, the typical practice is: the interconnection of various subnets in the same LAN and the routing between VLANs in the LAN are replaced by three-plex wide-layer switches, and only when cross-regional network access is to be realized between the LAN and the public network interconnection, the professional router is passed.

General understanding, FYI:

1. Layer 2 network can be implemented only through MAC addressing.

Only in large distributions is the use of three-layer exchange.

machine, very expensive. Ordinary switches are Layer 2 switches, which only recognize MAC addresses and do not know IP addresses (the computer is responsible for translation), so they are called Layer 2 switches and cannot be routed.

Layer 3 switches, Cisco, support VLAN, not only identify MAC addresses, but also put IP addresses in MAC frames.

Identify, route, powerful, complex, expensive, a switch can be divided into n network segments, but also n layer three switches distributed multiple network segments, called VLAN.

Our general understanding is Layer 3 routing, Layer 2**, if your switch is Layer 3, he remembers a route and then he will directly send it next time he doesn't need to learn, and the speed is much faster