VLANs, their Characteristics, and Implementation

1
VLANs, their Characteristics, and Implementation

• Hadi Ebrahimzadeh
• CS522
• Fall 2001

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.

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.

4
VLAN Benefits

• Increased performance
• Improved manageability
• Network tuning and simplification of software
  configurations
• Physical topology independence
• Increased security options

5
VLAN Limitations

• Broadcast limitations
• Device limitations
• Port constraints

6
VLAN Types

• Port-Based
• Policy-Based

7
Extending the VLAN

• Sometimes its necessary to create a VLAN that is
  geographically dispersed .
• Destination switch must clearly be able to
  identify the VLAN from which each frame
  originated .

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

9
VLANs and Spanning Tree

• To determine data flow between devices

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

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

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.
  10-12, January 1996
• Interconnections, Bridges and Routers Radia
  Perlman