SVI

An SVI (Switched Virtual Interface) is a virtual Layer 3 interface on a Layer 3 switch that acts as the default gateway for a VLAN. SVIs enable inter-VLAN routing without a separate physical router.

A Layer 3 switch can route traffic between VLANs internally using SVIs — one virtual interface per VLAN, each assigned an IP address that serves as the gateway for devices in that VLAN.

Without an SVI (or an external router), VLANs are completely isolated. VLAN 10 (192.168.10.0/24) and VLAN 20 (192.168.20.0/24) can't communicate at all. Add SVIs for both VLANs on a Layer 3 switch and it can route between them at wire speed, without traffic having to leave the switch.

This is the standard architecture for segmented enterprise networks: a Layer 3 core switch with SVIs for each VLAN handles inter-VLAN routing internally, while a separate firewall handles traffic leaving the network. You get the performance of local routing with the control of firewall policies at the perimeter.

SVIs also provide a management address for the switch itself — the interface you SSH or HTTPS to for switch management, typically on a dedicated management VLAN.