VLANs, their Characteristics, and Implementation Hadi Ebrahimzadeh CS522 Fall 2001
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.
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.
VLAN Benefits • Increased performance • Improved manageability • Network tuning and simplification of software configurations • Physical topology independence • Increased security options
VLAN Limitations • Broadcast limitations • Device limitations • Port constraints
VLAN Types • Port-Based • Policy-Based
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 .
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
VLANs and Spanning Tree • To determine data flow between devices
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
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
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