Virtual LAN
Meaning and Definition
VLAN is defined as multiple but separate broadcast domains
in a switch, which may be spread over to two or more switches. Following are
the benefits of VLANs:
- Segmentation
and Broadcast control
- Security
- Flexibility
Segmentation and Broadcast control
Each individual VLAN in a switch carries it’s own broadcast
domain, thus VLANs segments the switch into multiple broadcast domains.
Security
VLAN divides a switch into multiple broadcast groups. The
administrator controls each port and user. By implementing various access
policies, administrator can restrict hardware address, protocols, applications
etc.
Flexibility
The size of VLAN can easily be increased or decreased say
members of a VLAN can be shifted to some other VLAN.Flexibility also tells us
that more members can be included in the already existing VLAN.
When VLAN gets too big, we can create more VLAN to keep the
broadcast from consuming too much bandwidth.
Inter-Switch Link (ISL)
The important feature of a VLAN is that it restrict the
frame only within the same VLAN.Thus, each VLAN acts as an individual broadcast
domain.
When a VLAN is scaled to two or more that two switches we
call it a switch fabric. A frame traveling within the same VLAN but on
different switches need a frame identification technique known as frame
tagging. Frame tagging uniquely assigns a user-defined ID to each frame
known as VLAN ID or color.
ISL is a Cisco proprietary protocol used to
interconnect multiple switches and to maintain VLAN information as traffic goes
between switches. ISL operates in a point-to-point environment. The purpose of
ISL is to maintain VLAN information.
Functioning of ISL
When a frame traverses a trunked link, the VLAN tag is
removed before exiting trunked link. The second switch, where the frame reaches
must identify the VLAN ID. If the second switch is attached to a third switch
through a trunk link, the frame will be forwarded to the third trunk link.
Finally, when the frame reaches its destination to an access-link,
the switch removes the VLAN ID and the device receive the frame without VLAN
ID.
VLAN Trunking Protocol (VTP)
VTP is a layer 2 messaging protocol that maintains VLAN
configuration consistency throughout common administration domain by managing
the additions, deletions, and name changes of VLAN across networks.
VTP minimizes misconfigurations and configuration
inconsistencies that can cause problems, such as duplicate VLAN names or
incorrect VLAN-type specification.
It must be noted that configurations made to a single
switch, called as VTP server, are propagated across the trunk links to all
switches in the same VTP domain.
A VTP domain is one switch or several interconnected
switches sharing the same VTP environment. A switch cannot be the member of
more than one VTP domain at the same time.
VTP Modes of Operation
There are four different modes of operation within a VTP
domain. They are:
Server mode
Client mode
Transparent mode
|
Functions
|
Server
Mode
|
Client
Mode
|
Transparent
Mode
|
|
1.Default mode
|
Yes
|
No
|
No
|
|
2.Sends VTP advertisements
|
Yes
|
Yes
|
No
|
|
3.Forwards VTP advertisements
|
Yes
|
Yes
|
Yes
|
|
4.Syncronize VLAN configuration information with other
switches
|
Yes
|
Yes
|
No
|
|
5.VLAN configuration saved in NVRAM
|
Yes
|
No
|
Yes
|
|
6.Can create, modify, or delete VLANs using configuration
commands
|
Yes
|
No
|
Yes
|
VTP Pruning
For example, if a switch does not have any ports configured
for VLAN 5,and a broadcast is sent through out VLAN 5,the broadcast would not
traverse the trunk link to this switch.
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