Specifications

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Cisco 1751 Router Software Configuration Guide
OL-1070-01
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Routing Between Virtual LANs Overview
This chapter provides an overview of virtual LANs (VLANs). It describes the encapsulation protocols
used for routing between VLANs and provides some basic information about designing VLANs.
This chapter describes VLANs. It contains the following sections:
• What Is a VLAN?
• VLAN Colors
• Why Implement VLANs?
• Communicating Between VLANs
• Designing Switched VLANs
What Is a VLAN?
A VLAN is a switched network that is logically segmented on an organizational basis, by functions,
project teams, or applications rather than on a physical or geographical basis. For example, all
workstations and servers used by a particular workgroup team can be connected to the same VLAN,
regardless of their physical connections to the network or the fact that they might be intermingled with
other teams. Reconfiguration of the network can be done through software rather than by physically
unplugging and moving devices or wires.
A VLAN can be thought of as a broadcast domain that exists within a defined set of switches. A VLAN
consists of a number of end systems, either hosts or network equipment (such as bridges and routers),
connected by a single bridging domain. The bridging domain is supported on various pieces of network
equipment; for example, LAN switches that operate bridging protocols between them with a separate
bridge group for each VLAN.
VLANs are created to provide the segmentation services traditionally provided by routers in LAN
configurations. VLANs address scalability, security, and network management. Routers in VLAN
topologies provide broadcast filtering, security, address summarization, and traffic flow management.
None of the switches within the defined group will bridge any frames, not even broadcast frames,
between two VLANs. Several key issues need to be considered when designing and building switched
LAN internetworks.
• LAN Segmentation