Operation Manual - Link Layer Protocol
H3C SecPath F1800-A Firewall
Chapter 1 VLAN Configuration
4-1
Chapter 1 VLAN Configuration
1.1 Introduction to VLAN
1.1.1 The Potential Problems In LAN Interconnecting
The Ethernet is a kind of data network communication technology, which is based on
the shared communication medium of Carrier Sense Multiple Access with Collision
Detection (CSMA/CD). 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 topology) through the twisted
pairs, or connected together by the coaxial cables (with bus topology), all the hosts
interconnected to the shared physical media forms a physical collision domain, which
is usually regarded as a LAN segmentation.
Based on the Ethernet principles mentioned above, you can see that the following
problems may occur in connecting LAN through hub:
z
Severe collision
z
Flooding broadcast
z
Performance reduction
z
Unavailability of network
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 received frames.
For the received data frames, the switch will look up their destination MAC address in
the mapping table. If it can find the destination MAC address, the switch will send the
frame only to the corresponding port; if it cannot find the destination MAC address, the
switch will send the frame to all the ports.
In this way, the collision domains are separated in their own ports and will not be
extended 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.
Although the switch has solved the problem of severe collision caused by using hub, 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" (0xffffffffffff) as their destination MAC address, such as the 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 be degraded.
