Configuring Vlans; Overview; Vlan Frame Encapsulation - H3C S7500E-X Configuration Manual

Configuring VLANs

Overview

Ethernet is a family of shared-media LAN technologies based on the CSMA/CD mechanism. An
Ethernet LAN is both a collision domain and a broadcast domain. Because the medium is shared,
collisions and broadcasts are common in an Ethernet LAN. Typically, bridges and Layer 2 switches
can reduce collisions in an Ethernet LAN. To confine broadcasts, a Layer 2 switch must use the
Virtual Local Area Network (VLAN) technology.
VLANs enable a Layer 2 switch to break a LAN down into smaller broadcast domains, as shown
in Figure 36.
Figure 36 A VLAN diagram
Router
A VLAN is logically divided on an organizational basis rather than on a physical basis. For example,
you can assign all workstations and servers used by a particular workgroup to the same VLAN,
regardless of their physical locations. Hosts in the same VLAN can directly communicate with one
another. You need a router or a Layer 3 switch for hosts in different VLANs to communicate with one
another.
All these VLAN features reduce bandwidth waste, improve LAN security, and enable flexible virtual
group creation.

VLAN frame encapsulation

To identify Ethernet frames from different VLANs, IEEE 802.1Q inserts a four-byte VLAN tag
between the destination and source MAC address (DA&SA) field and the Type field.
Figure 37 VLAN tag placement and format
A VLAN tag includes the following fields:
•
TPID—16-bit tag protocol identifier that indicates whether a frame is VLAN-tagged. By default,
the TPID value 0x8100 identifies a VLAN-tagged frame. A device vendor can set the TPID field
VLAN 2
Switch A
VLAN 5
Switch B
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