7.2 Basic MLS Configuration
7.2.4 Common problem: creation of a null domain
When a Route Processor resides in a VTP domain other than the domain in which the switch resides, the switch can not multilayer switch frames for that router. There are several ways in which a Route Processor and switch can end up in different VTP domains:
  • You can purposely place both devices in separate domains.
  • You can misname or mistype the VTP domain when configuring either the switch or the Route Processor.
  • You can enter the MLS interface command prior to putting the interface in a VTP domain.

Enabling MLS on an interface before assigning the interface to a VTP domain places the interface in the null domain. When an interface resides in a null domain, it cannot participate in MLS with the switch. To remove the MLS interface from a null VTP domain, disable MLS on the interface.