Implementing VLANs
Benefits of local VLANs in the Enterprise Composite Network Model

Local VLANs are part of the Enterprise Composite Model design. VLANs used at the Access layer should extend no further then their associated distribution switch. Traffic is routed from the Local VLAN as it is passed from the Distribution Layer into the Core. This design can mitigate Layer 2 troubleshooting issues that occur when a single VLAN traverse switches throughout the Enterprise Campus Network. Implementing the Enterprise Composite Model using local VLANs provides the following benefits:

  • Deterministic traffic flow – The simple layout provides a predictable Layer 2 and 3 traffic path. In the event of a failure, which was not mitigated by the redundancy features, the simplicity of the model facilitates expedient problem isolation and resolution within the switch block.
  • Finite failure domain – If VLANs are local to a switch block and the number of devices on each VLAN is kept small, failures at Layer 2 are isolated to a small subset of users.
  • High availability – Redundant paths exist at all infrastructure levels. Local VLAN traffic on Access switches can be passed to the Building Distribution switches across an alternate Layer 2 path in the event of primary path failure. Redundant Layer 3 protocols can provide failover should the default gateway for the access VLAN fail. When both the STP instance and VLAN are contained to a specific Access and Distribution block, then Layer 2 and Layer 3 redundancy measures and protocols can be configured to failover in a coordinated manner.
  • Ease of Management – Local VLANs, typically confined to the Building Access submodule, are easier to plan and manage than VLANs spanning various switches and network areas. Also, local VLANs, when used in combination with dynamically assigned IP addresses, allow workstations to move from one VLAN to another with limited administrative overhead.