1. VLANs, their Characteristics, and Implementation
Hadi Ebrahimzadeh
CS522
Fall 2001
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2. What is a VLAN?
A VLAN is a collection of network ports that share a broadcast domain.
VLANs can also be extended physically separated locations.
VLANs are, by definition, unique broadcast domains.
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3. How do VLANs work?
A virtual internal bridge is created to connect a number of physical network segments together in each VLAN.
Most switches identify the VLAN by means of a numeric identifier.
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4. VLAN Benefits Increased performance Improved manageability
Network tuning and simplification of software configurations
Physical topology independence
Increased security options
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5. VLAN Limitations Broadcast limitations Device limitations
Port constraints
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6. VLAN Types
Port-Based
Policy-Based
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7. Extending the VLAN
Sometimes it’s necessary to create a VLAN that is geographically dispersed .
Destination switch must clearly be able to identify the VLAN from which each frame originated .
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8. Methods to determine VLAN membership
Implicit Tags
Based upon some attribute of the original frame
Explicit Tags
Frame itself carries an explicit identifier that determines VLAN membership.
Consumes bandwidth
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9. VLANs and Spanning Tree
To determine data flow between devices
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10. Distributing Routes Routing Information Protocol (RIP)
Distance Vector Protocol
Uses Bellman-Ford Algorithm
Makes its routing decisions based solely on distances (hops)
Open Shortest Path First (OSPF)
Link state routing protocols
Uses a replicated distributed database to calculate routes to all networks
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11. Challenging Problems of VLANs
VLANs require careful upfront network preparation.
Setting up VLANs requires a basic knowledge of routing protocols and protocol tagging
Network connectivity problems can be more difficult to isolate in a VLAN environment
The network administrator must keep up-to-date records of VLAN groups and switch configuration
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12. References Nortel Networks: Accelar 1000/8000 Configuration
Cisco Systems Inc., ``Virtual LAN Communications'', July 1995
Susan Biagi, "Virtual LANs," Network VAR v4 n1 p , January 1996
Interconnections, Bridges and Routers – Radia Perlman
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