Cisco IOS SVIs, also called VLAN interfaces, exhibit what might be considered an odd behavior: by default, an SVI will show an interface state of up but a line protocol state of down. Consider the following minimal configuration:
Switch(config)# vlan 10 Switch(config-vlan)# interface vlan10 Switch(config-if)# ip address 192.168.10.1 255.255.255.0 Switch(config-if)# ^Z Switch# show ip interface brief Interface IP-Address OK? Method Status Protocol Vlan1 unassigned YES unset administratively down down Vlan10 192.168.10.1 YES manual up down FastEthernet0/1 unassigned YES unset down down FastEthernet0/2 unassigned YES unset down down ...
This is because an SVI must meet all of the following conditions to transition to the full "up/up" state:
- Its VLAN must exist and be active in the VLAN database.
- At least one switched port in the VLAN (access or trunk) must be up.
- That port must be in the STP forwarding state.
Typically, a newly created VLAN will not yet have been assigned to any ports. Once it is, and provided at least one of those ports is operational, we see the SVI line protocol transition to the up state:
Switch(config)# interface f0/3 Switch(config-if)# switchport access vlan 10 Switch(config-if)# LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan10, changed state to up
Cisco refers to this behavior as autostate. While usually very handy, this behavior might not always be desirable. For example, an engineer might wish for an SVI to always remain up so that a can be reached via its associated IP address even if all ports associated with that VLAN are down (or if the VLAN itself has gone missing).
On some platforms (routers running 12.4T at least, possibly others), Cisco includes the ability to disable the autostate feature with the command
no autostate under SVI configuration. Sadly, this command seems only to be available on routers and not Catalyst switches, where one assumes it would be most useful.