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:
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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.
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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.
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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.
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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.