Switching Networking Theory

SVI

Switch Virtual Interfaces as Layer 3 gateways for VLANs on multilayer switches enabling inter-VLAN routing

SVI (Switched Virtual Interface)

  • Layer 3 logical interface that provides routing capabilities for VLANs on a multilayer switch
  • Each VLAN can have one SVI that acts as the default gateway for hosts in that VLAN
  • Exists in software only - no physical port required (hence “virtual”)
  • Enables inter-VLAN routing without requiring an external router

How SVIs Work

  • SVI automatically created when VLAN is configured, but remains in down/down state initially
  • SVI comes up when:
    • At least one active access port exists in the VLAN, OR
    • At least one active trunk port carries the VLAN
  • IP address assigned to SVI becomes the default gateway for that VLAN’s hosts
  • Switch performs routing between SVIs using its internal routing table

SVI Configuration 

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Switch(config)# interface vlan 10
Switch(config-if)# ip address 192.168.10.1 255.255.255.0
Switch(config-if)# no shutdown
Switch(config-if)# description Sales_Network_Gateway

SVI States and Troubleshooting

SVI State Line Protocol Common Causes
Up/Up Normal operation Active ports in VLAN exist
Up/Down VLAN exists but no active ports All ports shutdown or no ports assigned
Down/Down Interface shutdown Manual shutdown or VLAN doesn’t exist

Practical Use Cases

  • Inter-VLAN routing - Most common use case for connecting different VLANs
  • Network management - SVI provides IP connectivity to manage the switch remotely
  • DHCP relay - SVI can forward DHCP requests between VLANs and DHCP server
  • Small to medium networks where dedicated Layer 3 device isn’t cost-effective

SVI vs Physical Interface Routing

Aspect SVI Physical Interface
Hardware Required None (virtual) Physical router interface
Cost Lower (uses existing switch) Higher (separate router)
Performance Limited by switch CPU Dedicated routing hardware
Scalability Limited VLANs per switch More flexible routing options

Notes

  • Maximum SVIs limited by switch model - Check platform specifications (typically 256-1024)
  • SVI interface number must match existing VLAN number (interface vlan 10 requires VLAN 10)
  • Use show ip interface brief to verify SVI status and IP configuration
  • SVI will go down if VLAN is deleted - always verify VLAN exists first
  • For routing between SVIs to work, ip routing must be enabled globally
  • Consider SVI as the “router interface” for each VLAN segment
  • Best practice: Use SVI .1 address as default gateway (e.g., 192.168.10.1 for 192.168.10.0/24 network)
  • SVIs consume switch CPU resources - monitor performance in high-traffic environments