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This post is about VLAN Basics. Please see more details below.
What is VLAN technology?
Virtual Local Area Network (VLAN) technology divides a physical LAN into multiple broadcast domains (VLANs).
In March 1996, IEEE802.1 Internetworking Committee completed the initial VLAN standard revision. The new standard further enhances VLAN architecture, uniformly defines the tag format of different vendors in Frame-Tagging mode of suppliers in tag format, and stipulates the VLAN standard development trend within a period of time in the future. 802.1Q has been widely promoted in the industry. Later, IEEE issued 802.1Q in 1999 to standardize the VLAN implementation. 802.1Q promotes fast VLAN development because virtual networks do not depend on the sole vendor.
What is the difference between the VLAN data frame and the traditional Ethernet data frame?
IEEE 802.1Q adds the 4-byte 802.1Q tag between the Source Address field and the Length/Type field.
The VID field (VLAN ID) in a data frame identifies the VLAN that the data frame belongs to. The data frame can be only transmitted in the VLAN that the data frame belongs to.
What are the functions of VLAN technology?
The following figure shows the functions of VLAN technology.
Through VLAN assignment, hosts in a VLAN can communicate with each other, while hosts in different VLANs cannot communicate with each other. In this manner, broadcast packets are limited in a VLAN.
VLAN technology has the following advantages:
1. Limits broadcast domains: Broadcast domains are restricted in a VLAN, saving the bandwidth and improving the network processing capability.
2. Enhances LAN security: Packets in different VLANs are isolated. That is, users in a VLAN cannot communicate with users in another VLAN.
3. Improves network robustness: Faults are restricted in a VLAN, that is, faults in a VLAN do not affect other VLANs.
4. Flexibly creates virtual workgroups: VLAN technology can assign different users to different workgroups. Users in the same workgroup can be not restricted in a fixed physical position. Network construction and maintenance are convenient and flexible.
How is a VLAN data frame processed by different types of ports?
Which types of ports exist on the device?
Access port: connects to a host. It can connect to only the access link.
Trunk port: connects to another switch. It can connect to only the trunk link.
Hybrid port: connects to either a host or another switch. A hybrid port can connect to either an access link or a trunk link.
Note: Hybrid and trunk ports process the received data in the same manner. The hybrid port allows outgoing packets from multiple VLANs to be untagged, while the trunk port allows only outgoing packets from the default VLAN to be untagged.
Different switch ports process VLAN data frames in different manners.
1. Access port
2. Trunk port
3. Hybrid port
The following table describes packets processed by different ports.
Port Type | Untagged Incoming Frame Processing | Tagged Incoming Frame Processing | Outgoing Frame Processing |
Access port | Receives the frame and adds the default VLAN ID. | l Receives the frame when the frame's VLAN ID is the same as the default VLAN ID. l Discards the frame when the frame's VLAN ID is different from the default VLAN ID. | Removes the PVID and sends it out. |
Trunk port | l Receives the frame when the default VLAN ID is allowed by the port. l Discards the frame when the default VLAN ID is not allowed by the port. | l Receives the frame when the frame's VLAN ID is allowed by the port. l Discards the frame when the frame's VLAN ID is not allowed by the port. | l Removes the tag when the frame's VLAN ID is the same as the default VLAN ID and allowed by the port, and sends the frame out. l Retains the original tag when the frame's VLAN ID is different from the default VLAN ID and allowed by the port, and sends the frame out. |
Hybrid port | Sends the frame out when the frame's VLAN ID is allowed by the port. You can use commands to configure whether the device sends tagged frames. |
This issue describes the VLAN basics. For more information about VLAN technology, see the next issue.
To be continued...
Issue | Name | Description |
First issue | [All About Switches - Beginner] VLAN Basics | This issue describes the definition and purpose of VLAN technology and modes in which interfaces are added to VLANs. |
Second issue | This issue describes VLAN assignment modes and applicable scenarios, and mainly describes the configuration and scenario of the interface-based assignment. | |
Third issue | This issue describes the main technologies and application scenarios of inter-VLAN communication, including the VLANIF interface, sub-interface, and super-VLAN. This issue also describes common VLANIF interface configuration. | |
Fourth issue | This issue describes the main technologies and application scenarios of VLAN isolation, such as MUX VLAN and ACL. |
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