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ET 99.09 Cisco Semester 3
Chapter 3 VLANs
M. McGregor, Los Medanos College, Pittsburg, CA
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Introduction to VLANs
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What is a VLAN?
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Why create VLANs?
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Switches are the core of VLANs
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VLAN Switching and Filtering
Each switch has the intelligence to make filtering and
forwarding decisions by frame, based on VLAN metrics
defined by network managers, and to communicate this
information to other switches and routers within the
network.
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Frame filtering and tagging
The most common approaches for
logically grouping users into distinct
VLANs are:
• frame filtering
• frame tagging
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Frame Filtering
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Tradeoffs with frame filtering
• Filter-based VLANs do not scale well
because each frame has to be referenced to
a lookup table.
• The IEEE 802 committee has adopted
frame tagging as the standard because it is
more scalable.
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IEEE 802 VLAN Standardization
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Frame Tagging
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VLAN Flexibility
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Problems with broadcasts
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Problems with broadcasts
When no routers are placed between
switches, broadcasts (Layer 2 transmissions)
are sent to every switched port.
This is commonly referred to as a “flat”
network where there is one broadcast
domain across the entire network
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VLANs can control broadcasts
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VLANs can provide security
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VLANs provide flexibility
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Where are the routers?
Layer 3 communication, either embedded
in the switch or provided externally, is an
integral part of any high-performance
switching architecture.
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What are hubs good for?
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Leveraging your investment
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Port-Centric VLANs
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Port-centric VLANs
VLAN Membership by port maximizes
forwarding performance because:
 Users are assigned by port.
 VLANs are easily administered
 Maximizes security between VLANs
 Packets do not “leak” into other domains
 VLANs and membership are easily
controlled across network
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Static VLANs
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Dynamic VLANs
Dynamic VLANs are ports on a switch that
can automatically determine their VLAN
assignments.
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VLANs across backbones
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VLANs across backbones
• Fast Ethernet - ISL (Inter-Switch Link)
• FDDI - IEEE 802.10
• ATM - LAN Emulation (LANE)
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VTP - VLAN Trunk Protocol
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VTP Domain
VTP Configuration revision number
VLAN IDs (ISL)
Emulated LAN names (ATM)
802.10 SAID values (FDDI)
MTU
Frame format
VLAN configuration
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