Thanks to my readers, I often learn something completely new about the intricacies of Cisco IOS. The “VLAN Interface Status” post resulted in a comment about the SVI autostate concept, which is (not surprisingly) a somewhat muddy topic:
- In most cases, the SVI interface tracks the state of access and trunk ports using the VLAN. The details are well explained in the Understanding SVI Autostate section of the Cisco IOS documentation.
The important part of the SVI autostate calculation is the “port is in STP forwarding state for the VLAN” requirement. If a VLAN is not carried in a trunk port (for example, due to switchport trunk allowed configuration command), the trunk port’s status does not influence the autostate.
- In some IOS releases, you can exclude the individual physical ports from the autostate calculation with the switchport autostate exclude interface configuration command. Most commonly you’d want to exclude uplink ports on access switches.
- In some unspecified IOS releases (including 12.4T), you can use the (currently undocumented according to Command Lookup Tool) no autostate VLAN interface configuration command, which disables the autostate algorithm and makes the SVI interface permanently active.