The concept of a "virtual" LAN or VLAN is essentially that of an isolated broadcast domain that exists in LAN-switched environments. Ports sharing the same VLAN can switch packets between them at Layer 2 of the OSI stack. You can also think of a VLAN as an individual IP subnet. Cisco CCO describes the functions of VLANs as follows:
VLANs enable efficient traffic separation, provide better bandwidth utilization, and alleviate scaling issues by logically segmenting the physical local-area network (LAN) infrastructure into different subnets so that packets are switched only between ports within the same VLAN. When combined with central configuration management support, VLANs facilitate workgroups and client/server additions and changes.
This chapter identifies some of the key components of VLANs in Cisco-switched environments that relate to both performance management and fault management. We'll address the following VLAN features and characteristics:
Most of the discussion in this chapter applies to the Catalyst Series switches, and is basically not pertinent to the routers. Refer to Chapter 12, "Monitoring System Interfaces" for information on monitoring interfaces as it applies to the routers. VLAN interfaces act just like regular interfaces on routers in terms of monitoring.