Autostate
Hybrid and native switches have a feature called autostate. The feature is enabled by default and can
only be disabled in hybrid. In hybrid, logical VLAN interfaces configured on the
RSM/RSFC, MSFC, or Layer 3 module on the Catalyst 4000 rely on ports in Catalyst
OS to be active in the same VLANs before communication is possible. For example,
it is possible to configure a VLAN interface on an MSFC for VLAN 100 without any
switchports in Catalyst OS belonging to VLAN 100, or VLAN 100 even being defined
in Catalyst OS for that matter. Because this is possible, the Cisco IOS portion
of the hybrid configuration attempts to prevent a routing "black hole" by
placing the VLAN interface in a down/down state. After one or more active ports
or a trunk is configured in the same VLAN as the interface in Cisco IOS, the
VLAN interface changes to an up/up state. This checking mechanism is the result
of the autostate feature. One exception to this feature is for the VLAN assigned
to the management interface (sc0) on the switch. The sc0 interface can be shut
down administratively.
To further prevent black holes, the autostate feature on the
Catalyst 6000/6500 waits for the valid Layer 2 port(s) to transition into a
forwarding state before allowing the Layer 3 VLAN interface to transition to an
UP/UP state. The autostate on the Catalyst 6000/6500 feature began synchronizing
with spanning tree in this way starting in 5.5(10) and 6.1(1) Catalyst OS
software.
The commands in Example
7-11 disable autostate depending on the platform.
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