This transport domain is used to specify that particular SNMP messages are to be sent/received over a particular Virtual Private Network (VPN), implemented using MPLS (Multiprotocol Label Switching). The corresponding transport address is of type SnmpUDPVPNAddress. A VPN is defined as a set of sites with a common community of interest. Sites within an MPLS-based VPN often have private addresses which aren't accessible from outside of the VPN, and may be duplicates of private addresses used in other VPNs. To uniquely identify such a private address, it must be associated with a particular VPN routing/forwarding instance, also known as a VRF (VPN Routing and Forwarding table).