IP Forwarding: MLS SVI’s

Multilayer Switching allows for routing using a Switched Virtual Interface.  This is a logical virtual interface used as the routing gateway for a specific VLAN/Subnet.  Since this is a virtual port used for routing… the port itself can also go up and down given certain scenarios:

SVI Port can go down if:

  • the SVI is set to be ADMIN Down, the SVI interface will be shutdown
  • the corresponding vlan doesn’t actually exist on the device (SVI with no VLAN)
  • up, line protocol down – the VLAN exists but the port by which the VLAN traverses is on a port not in STP forwarding or not allowed on that trunk

Likewise the port can be in an up/up state indicating that all is well and the SVI should be capable of forwarding that traffic.

To avoid instances where you have an up interface but line protocol is down, one of the following must be true:

  • at least one physical trunk must be up,up and have the SVIs VLAN allowed, not pruned, and in STP forwarding.
  • at least one physical switch port is up/up and has the VLAN configured as an access or voice VLAN in the STP forwarding state.



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