VLANs (Virtual LANs)

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VLANs (Virtual LANs)

• CS 158B
• Elaine Lim
• Allison Nham

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LANs vs. VLANs

• In a traditional LAN...
• Users are grouped physically based on the hub
  they are plugged into
• Routers segment the LAN and provide broadcast
  boundaries
• In VLANs...
• Group users logically by function, department or
  application
• Configuration is done through special software

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A sample VLAN network
Source Cisco IOS Switching Services
Configuration Guide
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How VLANs work

• When a switch receives data from a workstation,
  it tags the data with a VLAN identifier that
  indicates which VLAN the data originally came
  from
• A packet can only travel from one broadcast
  domain to another if both domains have the same
  identifier
• To set up VLANs, we need VLAN-aware switching
  devices that must comply with IEEE 802.1Q
  standards
• Intelligent switches (operate at the MAC layer)
  or
• Routers (operate at the network layer of the OSI
  reference model)

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Static VLANs

• Defined
• Static VLANs are when ports on a switch are
  administratively assigned to a VLAN
• Benefits
• Secure easy to configure and monitor
• Works well in networks where moves are controlled

Source Cisco IOS Switching Services
Configuration Guide
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Dynamic VLANs

• Defined
• Switch ports can automatically determine a users
  VLAN assignment based on
• MAC address
• Protocol type
• Benefits
• Less administration when users are added or moved
• Centralized notification of unauthorized user

Source Cisco IOS Switching Services
Configuration Guide
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Frame Processing

• 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

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Frame Tagging

• A preferred way to implement VLANs
• Uniquely assigns a VLAN ID to each frame before
  it is forwarded across the backbone
• Is removed by the switch after frame exits the
  backbone

Source Cisco IOS Switching Services
Configuration Guide
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Traffic Between VLANs

• Switches do not forward frames between different
  VLANs. A router does this!!!
• Trunking a method that supports multiple VLANs
  that have members on more than one switch

• Two popular trunking protocols
• Cisco Inter-Switch Link (ISL)
• IEEE 802.1Q

Source How stuff works
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Inter-Switched Link (ISL)

• Cisco created ISL before the IEEE standardized a
  trunking protocol
• ISL is Cisco proprietary it can only be used
  between two Cisco switches
• ISL encapsulates each frame in an ISL header
  trailer

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IEEE 802.1Q

• 802.1Q is an open standard that can be used with
  multi-vendor switches
• 802.1Q does not encapsulate it adds an extra
  4-bytes header to the middle of the original
  Ethernet header
• 802.1Q forces a recalculation of FCS

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Disadvantage Advantages

• Disadvantage
• VLANs require significant overhead
• Advantages
• More Security
• Ease of administration
• Broadcast control
• Reduction in network traffic