Network design - confine VLANs to as few access switches as possible

When designing a network using the three tier model, with core, distribution, and access layers, it is best practice to keep VLANs confined to as few access switches as possible. In this way, broadcast domains remain small, and links between access and distribution switches and between distribution and core switches, do not become saturated with unnecessary broadcast traffic.

Ideally, a VLAN should be confined to a single access switch. This ideal situation however is usually not practical, so the following guidelines should be followed:

  1. Whenever possible, confine a VLAN to a single access switch
  2. Where this cannot be done, limit the spanning of VLANs to a single pair of distribution switches, or a single "switch block"
  3. If you have control over the applications being run within a spanned VLAN, try to keep that traffic to a minimum.

If none of the above can be done, ensure that the network capacities are large enough to handle traffic on spanned VLANs. This can be difficult, since changing traffic patterns can have more unpredictable effects in a network with spanned VLANs.

Ideally, the network should be redesigned in such a way so that the above principles can be achieved.