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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 8 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. Only one SVI can be associated with a VLAN, but you need to configure
an SVI for a VLAN only when you wish to route between VLANs, 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. In Layer 2 mode, SVIs provide IP host connectivity only to the system; in Layer 3 mode,
you can configure routing across SVIs.
Note
To use SVIs in Layer 3 mode, you must have the enhanced multilayer software image (EMI) installed
on your switch. All Catalyst 3550 Gigabit Ethernet switches ship with the EMI installed. Catalyst 3550
Fast Ethernet switches can be shipped with either the standard multilayer software image (SMI) or EMI
pre-installed. You can order the Enhanced Multilayer Software Image Upgrade kit to upgrade Catalyst
3550 Fast Ethernet switches from the SMI to the EMI.
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 ISL or 802.1Q
encapsulated trunk 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
“Configuring IP Addressing” section on page 22-4
.
SVIs support routing protocol and bridging configurations. For more information about configuring IP
routing, see
Chapter 22, “Configuring IP Unicast Routing,”
Chapter 24, “Configuring IP Multicast
Routing,”
and
Chapter 26, “Configuring Fallback Bridging.”
Routed Ports
A routed port is a physical port that acts like a port on a router; it does not have to be connected to a
router. A routed port is not associated with a particular VLAN, as is an access port. A routed port behaves
like a regular router interface, except that it does not support VLAN subinterfaces. Routed ports can be
configured with a Layer 3 routing protocol.
Note
To configure routed ports, you must have the EMI installed on your switch.
Configure routed ports by putting the interface into Layer 3 mode with the no switchport interface
configuration command. Then assign an IP address to the port, enable routing, and assign routing
protocol characteristics by using the ip routing and router protocol global configuration commands.
Caution
Entering a no switchport interface configuration command shuts the interface down and then
re-enables it, which might generate messages on the device to which the interface is connected.
Furthermore, when you use this command to put the interface into Layer 3 mode, you are deleting
any Layer 2 characteristics configured on the interface.
The number of routed ports and SVIs that you can configure is not limited by software; however, the
interrelationship between this number and the number of other features being configured might have an
impact on CPU utilization because of hardware limitations. For more information about feature
combinations, see the
“Optimizing System Resources for User-Selected Features” section on page 6-57
.
