Operation Manual – User Access
H3C SecPath Series Security Products
Chapter 4 VLAN Configuration
4-1
Chapter 4 VLAN Configuration
4.1 Introduction to VLAN
Ethernet is a kind of data network communication technology, which is based on the
shared communication medium of CSMA/CD (Carrier Sense Multiple Access with
Collision Detection). Under CSMA/CD, each node will use the shared medium to send
out frames in turn. Thus, in one moment, only one host can send out frames while other
hosts can only receive frames.
When many hosts are connected to the hub (with star architecture) through the twisted
pairs, or connected together by the coaxial cables (with bus architecture), all the hosts
interconnected to the shared physical media forms a physically collision domain, which
is usually considered as a LAN segmentation. According to the Ethernet principles
mentioned above, it can be concluded that the following problem exists in connecting
LAN through hub: excessive hosts can cause severe collision, broadcast storm and
affect the performance of the net or even make the net unusable.
The above problems can be solved by using the transparent bridge or LAN Switch to
interconnect the LANs. The switch establishes a MAC-PORT mapping table with the
source MAC addresses of frames received. For the received data frames, the switch
will look up their destination MAC addresses in the mapping table. If it can find a match,
it will only send the frames to the corresponding ports; if not, it will forward them to all
ports except for the receiving port. In this way, the collision domains are separated in
their own ports and will not extend to other ports. The switch, as a kind of transparent
device, does not change the source and destination addresses of the Ethernet frames,
but forwards them to the proper LAN segmentations. The switch usually uses the
special ASIC chip to implement the bridge switching. Although the switch has solved
the problem of severe collision from hub adoption, it still cannot separate the broadcast.
In fact, all the hosts (perhaps including many switches) interconnected by switches are
in one broadcast domain. For the broadcast packets with full "F" (0xffffff) as their
destination MAC address, such as ARP request packet, the switch will forward them to
all the ports. In this case, the broadcast storm will be caused and the performance of
the entire network will deteriorate.
The technology of VLAN (Virtual Local Area Network) comes into being to solve the
problem of broadcast restriction that switches cannot achieve in performing LAN
interconnection. By use of VLAN technology, one LAN is divided into several logical
"LANs" (VLANs), each indicating a broadcast domain. In each VLAN, the hosts can
communicate with each other just as they are in a LAN, but the VLANs cannot interact
with one another directly. Therefore, the broadcast packets are restricted in one VLAN,
as shown in the following figure:
