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What is VLAN, what does VLAN have effect

2018-02-23 15:37:30

What is VLAN?

A virtual LAN (VLAN) is a communication technology that divides a physical LAN logically into multiple broadcast domains.

In March 1996, the IEEE802.1Internetworking Committee ended the revision of the initial VLAN standards. The new standards further perfected the architecture of VLAN, unified the label formats of different manufacturers in Frame-Tagging mode, and formulated the development direction of VLAN standard in the future period. The standard of 802.1Q has been widely promoted in the industry. Later IEEE issued a draft of the 802.1Q protocol for standardized VLAN implementations in 1999. The emergence of 802.1Q broke the deadlock in which virtual networks depended on a single vendor and promoted the rapid development of VLAN from one side.

What is the difference between the VLAN data frame and the traditional Ethernet data frame?


IEEE 802.1Q is the official standard of VLAN. It adds 4 bytes of 802.1Q Tag on the basis of traditional Ethernet data frame (source MAC address field and protocol type field).

Among them, the VID (VLAN ID) field in the data frame is used to indicate the VLAN that the data frame belongs to, and the data frame can only be transmitted within the VLAN of the data frame.

What is the role of VLAN?

A picture can understand the role of VLAN.


As can be seen from the above diagram, by dividing different VLAN, the hosts in VLAN can communicate directly, while VLAN can not directly communicate with each other, thus limiting the broadcast message to one VLAN.

Here: the little editor summarizes the advantages of the next VLAN technology, and look at it together.

1. limited broadcast domain: the broadcast domain is limited to a VLAN, saving the bandwidth and improving the network processing capability.

2., enhance the security of LAN: the messages in different VLAN are isolated from each other during transmission, that is, users in a VLAN can't communicate directly with users in other VLAN.

3. improve the robustness of the network: the fault is limited to one VLAN, and the fault within this VLAN will not affect the normal work of other VLAN.

4., flexible construction of virtual working group: using VLAN can divide different users to different working groups. Users of the same working group do not have to be confined to a fixed physical scope, and network construction and maintenance are more convenient and flexible.

How does the VLAN data frame handle when it passes through the interfaces?

What types of interfaces are there on the device? Follow the little editor and look at it.


Access interface: an interface used to connect a user host on a switch, which can only connect to the access link (Access Link).

Trunk interface: an interface that is used to connect to other switches on a switch, which can only connect to the Trunk Link.

In addition, there is an interface called the Hybrid interface, which is the interface that can connect both the user host and the other switches on the switch. The Hybrid interface can connect both the access link and the trunk link.

Note: the processing method is the same when the Hybrid interface and the Trunk interface receive the data. The only difference is that when sending data, the Hybrid interface can allow multiple VLAN messages to be sent without labels, while the Trunk interface allows only the default VLAN messages to be sent without labels.

The type of interface is different, and the process of processing VLAN data frames is different. The specific processing methods are as follows:

1, Access interface


2, Trunk interface


3, Hybrid interface


In order to help the young partners better understand the processing of the message, we can also learn through the following forms!!!

Interface type

      Receive a message without Tag

 Receive a message with Tag

Transmission frame processing

 Access Interface

 Receive the message and hit the default VLAN ID.  

Contrast VLAN ID with default VLAN ID

L phase at the same time, receiving the message.

L is not at the same time, discarding the message.


The PVID Tag of the frame is stripped first and then sent again.

  Trunk Interface

Hit the default VLAN ID

L VLAN ID when the default in the list of VLAN ID allows when receiving the message.

When the default VLAN ID l was not in the list of VLAN ID allows in, discarding the message.

When the VLAN ID L in the list of VLAN ID interface allows the when receiving the message.

L when VLAN ID was not in the list of VLAN ID interface allows the while discarding the message.


When the VLAN ID L and default VLAN ID, and the interface allows the VLAN ID, remove the Tag, send the message.

L ID and VLAN VLAN when the default ID is different, and the interface allows the VLAN to maintain the original ID, Tag, send the message.

Hybrid Interface

The message is sent when the VLAN ID is the VLAN ID that the interface is allowed to pass. You can use the command to set up whether to carry Tag when sending.

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