VLANs
Semester 3, Chapter 3
Linda Stacy
Table of Contents
Go There!
Virtual LANs (VLANs)
Go There!
Segmentation with Switching
Architecture
Go There!
VLAN Implementation
Go There!
Benefits of VLANs
Virtual LANs
(VLANs)
Table of Contents
Existing Shared LAN Configurations
In a typical shared LAN...
 Users are grouped physically based on the hub they are
plugged into
 Routers segment the LAN and provide broadcast firewalls
In VLANs...
 you can group users logically by function, department or
application in use
 configuration is done through proprietary software
Segmentation with
Switching Architecture
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Grouping Users
VLANs can logically segment users into different subnets
(broadcast domains)
 Broadcast frames are only switched between ports on the
switch or switches with the same VLAN ID.
 Users can be logically group via software based on:

port number
MAC address
protocol being used
application being used
Differences between LANs & VLANs
VLANs...
 work at Layer 2 & 3
 control network
broadcasts
 allow users to be
assigned by net admin.
 provide tighter network
security. How?
VLANs Across the Backbone



VLAN configuration needs to
support backbone transport
of data between
interconnected routers and
switches.
The backbone is the area
used for inter-VLAN
communication
The backbone should be
high-speed links, typically
100Mbps or greater
Router’s Role in a VLAN
A router provides connection between
different VLANs
For example, you have VLAN1 and
VLAN2.
 Within the switch, users on separate VLANs cannot talk to
each other (benefit of a VLAN!)
 However, users on VLAN1 can email users on VLAN2 but
they need a router to do it.
How Frames are Used in a VLAN
Switches make filtering
and forwarding decisions
based on data in the
frame.
There are two techniques
used.


Frame Filtering--examines
particular information about
each frame (MAC address or
layer 3 protocol type)
Frame Tagging--places a
unique identifier in the header
of each frame as it is
forwarded throughout the
network backbone.
More on Frame Tagging
Frame Tagging...
 is specified by IEEE 802.1q which states
frame tagging is the preferred way to
implement VLANs
 uniquely assigns a VLAN ID to each frame
before it is forwarded across the backbone.
 is understood by switches prior to any
broadcasts or transmission to other
switches or routers
 places a tag in the frame...thus, frame
tagging. So what layer?
 is removed by the switch after frame exits
the backbone and before frame is
forwarded to the end station
VLAN Implementation
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Ports, VLANs, and Broadcasts
Three methods for implementing VLANs
 Port-Centric
 Static
 Dynamic
Each switched port can be assigned to a
VLAN. This...
 ensures ports that do not share the same VLAN do not
share broadcasts.
 ensures ports that do share the same VLAN will share
broadcasts.
Benefits of Port-Centric VLANs

All nodes in the same
VLAN are attached to the
same router interface.
(Note: curriculum says “switched port”)

Makes management
easier because...
Users are assigned by
router port
VLANs are easy to admin.
provides increased security
packets do not “leak” into
other domains
3 Port-Centric
VLANs
Static VLANs
Defined
 Static VLANs are when
ports on a switch are
administratively assigned
to a VLAN
Benefits
 can be assigned by port,
address, or protocol type
 secure, easy to configure
and monitor
 works well in networks
where moves are
controlled
Dynamic VLANs

Defined
 Switch ports can automatically determine a
user’s VLAN assignment based on either/or:
 MAC
 logical address
 protocol type
 When a station is initially connected to an
unassigned port, the switch checks an entry in
the table and dynamically configures the port
with the right VLAN

Benefits
 less administration (more upfront) when users
are added or move
 centralized notification of unauthorized user
Benefits of VLANs
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VLANs Make Changes Easier
Traveling Users
 20% to 40% of work force moves every year
net admin’s biggest headache
largest expense in managing networks. Moves may require...
 recabling
 readdressing and reconfiguration

VLANs provide a way to control these costs. As long as the
user still belongs to the same VLAN...
simply configure the new switch port to that VLAN
router configuration remains intact
VLANs Control Broadcasts
Routers provide an effective
firewall against broadcasts
 Adding VLANs can extend a
router’s firewall capabilities to
the “switch fabric”
 The smaller the VLAN, the
smaller the number of users
that are effected by
broadcasts

VLANs Improve Security
Shared LANs are easy to penetrate...simply plug into the
shared hub.
 VLANs increase security by ...

restricting number of users in a VLAN
preventing user access without authorization
configuring all unused ports to the “Disabled” setting
control access by
 addresses
 application types
 protocol types
VLANs Save Money
Hub Replacement &
Segmentation
 The ports on a non-intelligent
hub can only be assigned one
VLAN.
 Replacing hubs with switches
is relatively cheap compared to
the benefit gained.
 In the graphic, replacing the
core hub in an extended star
topology with a VLAN capable
switch effectively
microsegments one shared
LAN into six.
Table of Contents
End Slide Show