In a switched network, VLANs separate devices into different collision domains. Additionally, VLANs are also used to separate devices into different subnets. Devices within a VLAN can communicate with each other without the need for routing, assuming that these devices reside within the same subnet. However, devices in separate VLANs require a routing device to communicate with one another. Traditionally, IP routing and any Layer 3 functions, such as LAN default gateway functionality, were primarily implemented on routers. This entailed using multiple physical router interfaces (in different VLANs) to provide gateway and routing functionality between different VLANs, or using a single physical router interface and then creating multiple subinterfaces, each serving as the default gateway for the specified VLAN, thus allowing devices in different VLANs to communicate.
However, in modern networks, these functions are performed by Multilayer switches. MLS is described in additional detail later in this chapter. Multilayer switches support the configuration of Switched Virtual Interfaces (SVIs), which represent VLANs and allow the switch to serve as the default gateway for the VLAN. Although the SVI represents a VLAN, it is not automatically configured when a Layer 2 VLAN is configured on the switch.
Likewise, the switch will not automatically create a VLAN if you configure an SVI. The only exception to this rule is the SVI for VLAN 1, which is the default VLAN. This SVI is automatically created by the software to allow for remote administration of the switch; however, it defaults to an administratively shutdown state and must be manually brought up and configured with Layer 3 addressing. Additionally, the switch should be configured with the correct default gateway, or on Multilayer switches, IP routing should be enabled.
A Switched Virtual Interface is a very resilient interface. In order for an SVI to be placed into the up/up state, the following conditions must be met:
- The VLAN exists and is active in the VLAN database of the switch
- The VLAN interface is not administratively down
- At least one Layer 2 (access port or trunk) port exists, has a link up on this VLAN
- At least one Layer 2 (access port or trunk) is in the STP forwarding state
In addition to SVIs which provide default gateway functions for VLANs, Multilayer switches also support IP addressing configuration on physical interfaces. However, it is important to remember that switches do not route non-IP packets between VLANs and routed ports. However, you can forward these non-IP packets using fallback bridging, which is described in detail in the current SWITCH guide which is available online.