Implementing VLANs
What is a local VLAN?

In the past, network designers attempted to implement the 80/20 rule when designing networks. The rule was based on the observation that, in general, 80 percent of the traffic on a network segment went between local devices, and only 20 percent of the traffic was destined for remote network segments. Network designers now consolidate servers in central locations on the network, and provide access to external resources such as the Internet through one or two paths on the network, as the bulk of traffic now traverses a number of network segments. Therefore the paradigm has changed more to a 20/80 where the greater flow of traffic leaves the local segment.

Additionally, the concept of end-to-end VLANs was very attractive when IP address configuration was a manually administered and burdensome process; therefore, anything that reduced this burden as users moved between networks was a good thing. But given the ubiquity of DHCP, the process of configuring IP at each desktop is no longer a significant issue. As a result there are few benefits to extending a VLAN throughout an enterprise. It is often more efficient to group all users on a set of geographically common switches into a single VLAN regardless of the organizational function of those users, especially from a troubleshooting perspective. VLANs that have boundaries based upon campus geography rather than organizational function are called "local VLANs."

Here are some local VLAN characteristics and usage guidelines:

  • Local VLANs should be created with physical boundaries in mind, rather than job functions of the users on the end devices.
  • Traffic from a local VLAN is routed to reach destinations on other networks.
  • A single VLAN does not extend beyond the Building Distribution submodule.
  • VLANs on a given access switch should not be advertised to all other switches in the network.