12-5
Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter12 Configuring Interface Characteristics
Understanding Interface Types
Switch Virtual Interfaces
A switch virtual interface (SVI) represents a VLAN of switch ports as one interface to the routing or
bridging function in the system. You can associate only one SVI with a VLAN. You configure an SVI
for a VLAN only to route between VLANs, to fallback-bridge nonroutable protocols between VLANs,
or to provide IP host connectivity to the switch.
By default, an SVI is created for the default VLAN (VLAN 1) to permit remote switch administration.
Additional SVIs must be explicitly configured.
Note You cannot delete interface VLAN 1.
SVIs provide IP host connectivity only to the system. In Layer3 mode, you can configure routing across
SVIs. Although the switch supports a total or 1005 VLANs (and SVIs), the interrelationship between the
number of SVIs and routed ports and the number of other features being configured might impact CPU
performance because of hardware limitations. See the “Configuring Layer 3 Interfaces” section on
page 12-25 for information about what happens when hardware resource limitations are reached.
SVIs are created the first time that you enter the vlan interface configuration command for a VLAN
interface. The VLAN corresponds to the VLAN tag associated with data frames on an encapsulated trunk
port or the VLAN ID configured for an access port. Configure a VLAN interface for each VLAN for
which you want to route traffic, and assign it an IP address. For more information, see the “Manually
Assigning IP Information” section on page3-16.
Note When you create an SVI, it does not become active until it you associate it with a physical port.
SVIs support routing protocols and bridging configurations. For more information about configuring IP
routing, see Chapter 38, “Configuring IP Unicast Routing,” Chapter46, “Configuring IP Multicast
Routing,”and Chapter48, “Configuring Fallback Bridging.”
Note The IP base image supports static routing and RIP; for more advanced routing or for fallback bridging,
you must have the IP services image installed.

SVI Autostate Exclude

The line state of an SVI with multiple ports on a VLAN is in the up state when it meets these conditions:
•The VLAN exists and is active in the VLAN database on the switch.
•The VLAN interface exists and is not administratively down.
•At least one Layer 2 (access or trunk) port exists, has a link in the up state on this VLAN, and is in
the spanning-tree forwarding state on the VLAN.
Note The protocol link state for VLAN interfaces come up when the first switchport belonging to the
corresponding VLAN link comes up and is in STP forwarding state.
The default action, when a VLAN has multiple ports, is that the SVI goes down when all ports in the
VLAN go down. You can use the SVI autostate exclude feature to configure a port so that it is not
included in the SVI line-state up-an- down calculation. For example, if the only active port on the VLAN