This section answers questions concerning VLANs and spanning tree on Cisco Catalyst IOS devices.
There is no good way to collect VLAN utilization, for the following reasons:
Not all switch ports in a VLAN see all traffic in the VLAN.
Not all traffic for a VLAN traverses trunk ports.
Not all traffic for a VLAN hits a switch backplane.
Therefore, there is no single point at which you can measure traffic to accurately reflect the aggregate bandwidth consumed for a VLAN.
There are more useful methods for measuring capacity consumption. For instance, measuring broadcast and multicast traffic from a specific port can provide a benchmark for rising capacity needs. For a VLAN, this method is useful ...