Viewing Active VLANs and VLAN Configuration

The device viewer VLANs page (Device Viewer > Switch > VLANs) provides a summary table for all of the VLANs provisioned in the selected switch.

The Active VLANs (Device Viewer > Switch > VLANs > Active VLANs) table provides some important information for checking the switch's VLAN configuration and its status in the network.

The Active VLANs table lists all VLANs being supported by the device, including the root bridge for each VLAN and elements such as the Root Priority, Switch Priority, Root Cost, and the Spanning Tree Protocol. Root Bridge Priority values are used in the election process of a root bridge for a particular VLAN.

A root bridge is selected by setting a switch's root priority value to a lower value in comparison to other switches. The root bridge priority value defaults to 32768 for most platforms and the maximum value is 65535; the minimum value is 0. The bridge priority value is combined with the MAC address ID for the switch to determine the spanning-tree root bridge for the network. This resulting value propagates through the switched network in Bridge Protocol Data Units (BPDUs) from the root bridge, to ensure that the devices in the switched network agree on the identity of the root bridge.

Should all switches in the network retain the same value, an election takes place in which the switch with the lowest MAC address becomes the root bridge. Many older Ethernet switches may have lower Ethernet MAC address values and may thus be automatically elected as the root bridge for many VLANS in the network, even though the switch will not have the processing or memory to handle the load. To ensure the 'correct' switch is elected as the root, the best practice is to set the desired core switch's bridge priority to a relatively low value such as 8000; then, a second root bridge is chosen as a backup root and its priority set to a slightly higher value.

Listed on the Active VLANs page, the switch priority of a VLAN is the value defined in the local switches' configuration as the candidate value for election as the root.

The root cost value in the table is the cumulative cost of all links in the current VLAN leading to the root bridge. VLAN IDs that show a value of 0 are not participating in the spanning tree.

The Spanning Tree Protocol column reports the version of the spanning tree protocol being run on each switch interface. A normal value for this column is ieee8021d.