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.