SVIs

An SVI, or Switched Virtual Interface, is a virtual Layer 3 interface configured on a multilayer switch to provide IP routing for a specific VLAN. Each SVI corresponds to a VLAN ID and is used to enable inter-VLAN routing within the switch fabric, eliminating the need to route traffic to an external router for intra-network traffic.

SVIs are created by defining an interface VlanX and assigning it an IP address, which becomes the default gateway for devices in that VLAN. The VLAN must exist in the switch’s VLAN database, and IP routing must be enabled on the device (for example, ip routing on Cisco IOS).

With SVIs, endpoints in different VLANs can reach each other via the switch’s internal routing table. This approach is typical on multilayer switches and is often compared with router-on-a-stick, where a separate router handles inter-VLAN routing using subinterfaces on a trunk link.

Advantages include centralized policy, simplified management, and potential performance benefits on capable switches. Limitations include hardware resource constraints, as the number of SVIs is limited by memory and CPU; not all switches support SVIs; and some deployments rely on external routers or software-based routing. SVIs can participate in routing protocols (such as OSPF, EIGRP, or BGP) and support features like DHCP relay, VRRP/HSRP for gateway redundancy, and SNMP for management.

In summary, SVIs are a key feature in Layer 3 switching, enabling scalable, VLAN-based routing within a single device and are commonly used in enterprise networks to provide efficient inter-VLAN routing.