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eSight V300R009C00 Operation Guide 10

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
VLAN Overview

VLAN Overview

Virtual Local Area Networks (VLANs) are created on a physical LAN to separate the LAN into multiple broadcast domains. The hosts in a VLAN can directly communicate with each other, whereas the hosts in different VLANs cannot. This efficiently suppresses broadcasting packets within a VLAN.

VLAN Sources

Ethernet uses shared communication media to transmit data based on Carrier Sense Multiple Access (CSMA)/Collision Detect (CD). Numerous hosts may cause serious conflicts, excessive broadcast packets, and poor performance, and even network unavailability. Using switches to connect LANs reduces conflicts but cannot separate broadcast packets. VLAN technology divides a physical LAN into multiple VLANs to isolate broadcast domains. Hosts within a VLAN can only directly communicate with hosts in the same VLAN. They must use a router to communicate with hosts in other VLANs.

VLAN Advantages

  • Broadcast domain restriction: The broadcast domain is restricted to a VLAN, which saves bandwidth and improves a network's processing ability.
  • Enhanced LAN security: Packets in a VLAN are separated from another VLAN. This makes users between VLANs unavailable to communicate with each other directly.
  • Improved network robustness: Faults are limited to a VLAN. Faults in a VLAN will not affect the normal operation of other VLANs.
  • Allows flexible construction of virtual groups. With VLAN technology, hosts in different geographical locations can be grouped together, simplifying network construction and maintenance.

VLAN Port Types

According to the port capabilities to identify a VLAN frame, the ports can be classified into the following types:

  • Access port: An access port is used to connect a host and can connect to only an access link. Only the unique VLAN ID can pass a port and the VLAN ID must be the same as the default VLAN ID of the port. Ethernet frames sent by an access port to peer devices do not carry any tag.
  • Trunk port: A trunk port is used to connect to other hosts and can connect only a trunk link. A trunk port permits the tagged frames of multiple VLANs to pass through.
  • Hybrid port: A hybrid port can connect to a host or ports of other switches. A hybrid port can connect to either an access link or a trunk link. A hybrid port permits the frames of multiple VLANs to pass through. It can strip tags off certain frames in the outgoing direction of the port.

Basic Concepts of the VLAN

  • Port Default VLAN ID (PVID): Ports of each type can be assigned with a default VLAN whose VLAN ID is called PVID. The meaning of a default VLAN varies with the port type. To increase the efficiency in processing frames, all the data frames transmitted on a switch carry VLAN tags and are processed uniformly. When a data frame without a VLAN tag reaches a port on the switch that is configured with the PVID, the PVID of the port is added to the data frame. If the data frame carries a VLAN tag, the switch does not add a VLAN tag to the frame even if the port is configured with the PVIDs. The switch processes frames differently according to the port type. For details, see Table 12-13.
  • Voice VLAN: Two data flows, including voice data and non-voice data, exist on the network. Voice data has higher priority than non-voice data during transmission, which reduces the delay and packet loss phenomena that may arise from the transmission. Ports that enable voice VLAN determine whether a data flow is voice data based on the source MAC address field carried in the data flow. If yes, the ports transmit the voice data through the Voice VLAN, which simplifies user configuration and facilitates user management over voice data.

  • Organizationally Unique Identifier (OUI): The OUI is the first 24 binary bits of an MAC address, used to identify devices from different vendors. The OUI can be determined by the calculation of the 48-bit MAC address and mask. If the first 24 bits of an MAC address in a data flow match the OUI, ports that enable voice VLAN recognize the data flow as voice data and the access device as voice device.
  • VLANIF Layer 3 switching technology combines routing and switching technologies, enables the routing function within switches, and improves network performance. A Layer 3 switch generates a mapping table of MAC addresses and IP addresses after transmitting the first data flow through the routing table. When recognizing a data flow as the same data flow, the switch automatically makes the data flow pass through at the Layer 2 rather than the Layer 3, which eliminates the network delay arising from the routing process and improves the package forwarding efficiency. To ensure the normal forwarding of the first data flow, the routing table must have correct routing entries. Layer 3 interfaces and routing protocols must be deployed on the Layer 3 switch to enable Layer 3 routing reachability. VLANIF interfaces are therefore introduced. A VLANIF interface is a logical Layer 3 interface that can be deployed on a Layer 3 switch or router.

Principle of the VLAN Communication

To increase the efficiency in processing frames, all the data frames transmitted on a switch carry VLAN tags and are processed uniformly. When a data frame without a VLAN tag reaches a port on the switch that is configured with the PVID, the PVID of the port is added to the data frame. If the data frame carries a VLAN tag, the switch does not add a VLAN tag to the frame even if the port is configured with the PVIDs. The switch processes frames differently according to the port type. The following table describes the frame processing on the ports of different types.

Table 12-13 Frame processing on the ports of different types

Port Type

Processing for a Received Packet Without a Tag

Processing for a Received Tagged Packet

Frame Processing

Access port

Accept the packet and add the default VLAN tag to the packet.

  • Accept the packet when the VLAN ID of the packet is the same as the default VLAN ID of the port.
  • Discard the packet when the VLAN ID of the packet is different from the default VLAN ID of the port.

Strip off the PVID fag of the frame for transmission.

Trunk port

  • Add the default VLAN ID. Accept the packet when the default VLAN ID of the packet is in the list of VLAN IDs allowed to pass through the port.
  • Add the default VLAN ID. Discard the packet when the default VLAN ID of the packet is not in the list of VLAN IDs allowed to pass through the port.
  • Accept the packet when the VLAN ID of the packet is in the list of VLAN IDs allowed to pass through the port.
  • Discard the packet when the VLAN ID of the packet is not in the list of VLAN IDs allowed to pass through the port.
  • Remove the tag and send the packet when the VLAN ID of the packet is the same as the default VLAN ID of the port and the VLAN ID is allowed to pass through the port.
  • Retain the tag and send the packet when the VLAN ID of the packet is different from the default VLAN ID of the port and the VLAN ID is allowed to pass through the port.

Hybrid port

Send the packet when the VLAN ID of the packet is in the list of VLAN IDs allowed to pass through the port. You can run commands to configure whether to carry tags during packet transmission.

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Updated: 2019-09-07

Document ID: EDOC1100011877

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