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).