Network Analysis Tip #1: VLAN With No Active Ports

Why is this important?

A device configured with a VLAN and has no active ports may be the result of a misconfiguration. Another source might be the re-deployment of a switch with a previous configuration to a different part of the network, resulting in VLANs that appear where they shouldn’t.

Manual determination:

Finding VLANs with no active ports is very easy using manual methods, The command in Cisco IOS:

If there are no ports in the VLAN on this switch, then the Ports list will be empty, such as VLAN 3, named ‘Remote’, above. A single switch with no ports in the spanning tree for a VLAN may be properly configured and is waiting for a port to become active in that VLAN, either due to manual configuration or dynamic VLAN operation. In the case of manual configuration, any switch that has a VLAN with no ports may be configured to prune the VLAN on the trunk in order to minimize trunk utilization (remember, broadcasts are flooded to the entire VLAN, which includes switches that have no ports in the VLAN because the originating switch doesn’t know there are no ports in the VLAN of the destination switch.

Automatic determination:

The same data that is obtained via manual methods can be obtained with SNMP. A bit of analysis is used to determine that there are no ports in a defined VLAN on a switch. The screen shot to the right shows a switch with several empty VLANs. Further investigation showed that this switch was the only one in each of these VLANs. These VLANs existed on other switches, but this switch was isolated from them by a router. The configuration had been copied from the other switches without thought about which VLANs would actually be needed. It would also make sense to give the VLANs names that aided in future maintenance and troubleshooting.

Further reference:

Cisco Whitepaper: “Best Practices for Catalyst 6500/6000 Series and Catalyst 4500/4000 Series Switches Running Cisco IOS Software” at http://www.cisco.com/en/US/products/hw/switches/ps700/products_white_paper09186a00801b49a4.shtml.