Virtual LANs
Figure
2.85 shows a switched LAN in an engineering firm in which 10 stations are
grouped into three LANs that are connected by a switch. The first four
engineers work together as the first group, the next three engineers work
together as the second group, and the last three engineers work together as the
third group. The LAN is configured to allow this arrangement.
1. Membership
Vendors
use different characteristics such as port numbers, MAC addresses, IP
addresses, IP multicast addresses, or a combination of two or more of these.
2. Port Numbers
Some VLAN
vendors’ uses switch port numbers as a membership characteristic. For example,
the administrator can define that stations connecting to ports 1, 2, 3, and 7
belong to VLAN 1; stations connecting to ports 4, 10, and 12 belong to VLAN 2;
and so on.
3. MAC Addresses
Some VLAN
vendors use the 48-bit MAC address as a membership characteristic. For example,
the administrator can stipulate that stations having MAC addresses E21342A12334
and F2A123BCD341belong to VLAN 1.
4. IP Addresses
Some VLAN
vendors use the 32-bit IP address as a membership characteristic. For example,
the administrator can stipulate that stations having IP addresses 181.34.23.67,
181.34.23.72, 181.34.23.98, and 181.34.23.112 belong to VLAN 1.
5. Multicast IP Addresses
Some VLAN
vendors use the multicast IP address as a membership characteristic.
Multicasting at the IP layer is now translated to multicasting at the data link
layer.
6. Combination
Recently,
the software available from some vendors allows all these characteristics to be
combined. The administrator can choose one or more characteristics when
installing the software.
7. Configuration
Stations
are configured in one of three ways: manual, semiautomatic, and automatic.
a. Manual Configuration
In a
manual configuration, the network administrator uses the VLAN software to
manually assign the stations into different VLANs at setup. Later migration
from one VLAN to another is also done manually. Note that this is not a
physical configuration; it is a logical configuration.
b. Automatic Configuration
In an
automatic configuration, the stations are automatically connected or
disconnected from a VLAN using criteria defined by the administrator. When a
user changes the project, he or she automatically migrates to a new VLAN.
c. Semiautomatic Configuration
A
semiautomatic configuration is somewhere between a manual configuration and an
automatic configuration. Usually, the initializing is done manually, with
migrations done automatically.
8. Time-Division Multiplexing
(TDM)
In this
method, the connection (trunk) between switches is divided into timeshared
Channels. For example, if the total number of VLANs in a backbone is five, each
trunk is divided into five channels. The traffic destined for VLAN 1 travels in
channel, the traffic destined for VLAN 2 travels in channel 2, and so on. The
receiving switch determines the destination VLAN by checking the channel from
which the frame arrived.
9. Advantages
There are
several advantages to using VLANs.
· Cost and
Time Reduction
· Creating
Virtual Work Groups
· Security
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