H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 57)

7-2
Figure 7-1 A VLAN diagram
VLAN 2
VLAN 5
Switch B Switch A
Router
A VLAN is not restricted by physical factors, that is to say, hosts that reside in different network
segments may belong to the same VLAN, users in a VLAN can be connected to the same switch, or
span across multiple switches or routers.
VLAN technology has the following advantages:
2) Broadcast traffic is confined to each VLAN, reducing bandwidth utilization and improving network
performance.
3) LAN security is improved. Packets in different VLANs are isolated at Layer 2. That is, users in a
VLAN cannot communicate with users in other VLANs directly, unless routers or Layer 3 switches
are used.
4) A more flexible way to establish virtual workgroups. With VLAN technology, a virtual workgroup can
be created spanning physical network segments. That is, users from the same workgroup do not
have to be within the same physical area, making network construction and maintenance much
easier and more flexible.
VLAN Fundamental
To enable packets being distinguished by the VLANs they belong to, The VLAN tag fields used to
identify VLANs are added to packets. As common switches operate on the data link layer of the OSI
model, they only process data link layer encapsulation information and the VLAN tag thus needs to be
inserted to the data link layer encapsulation.
The format of the packets carrying the VLAN tag fields is defined in IEEE 802.1Q, which is issued by
IEEE in 1999.
In the header of a traditional Ethernet data frame, the field following the destination MAC address and
the source MAC address is the Type field, which indicates the upper layer protocol type.
Figure 7-2
illustrates the format of a traditional Ethernet frame, where DA stands for destination MAC address, SA
stands for source MAC address, and Type stands for the upper layer protocol type of the frame.
Figure 7-2 The format of a traditional Ethernet frame