An SD-WAN Service Provider typically builds point-to-point paths called Tunnel Virtual Connections (TVCs) across the various Underlay Connectivity Services that compose an SD-WAN Service. Each TVC is built over a single Underlay Connectivity Service and has a well-defined set of characteristics, many of which are inherited from the Underlay Connectivity Service. One of the important functions of the SD-WAN Edge is to select a TVC over which to forward each ingress IP Packet.

TVCs are internal to the Service implementation and therefore do not have Service Attributes. However, TVCs play an important part of the packet forwarding function in the SD-WAN Edge, so a brief discussion is warranted.

Characteristics of a TVC include:

  • A TVC is Public or Private based on the Underlay Connectivity Service that it is built on.
  • A TVC has a charge model of fixed-rate or usage-based that reflects the Underlay Connectivity Service that it is built on.
  • A TVC can be encrypted or unencrypted.
  • A TVC has performance and bandwidth constraints and behaviors that derive from the Underlay Connectivity Service that it is built on.

Also, by building point-to-point TVCs, a Service Provider creates a virtual topology that can be different from the physical topology of the Underlay Connectivity Service. For example, if one of the Underlay Connectivity Services is an EP-LAN service connecting all of the SD-WAN Edges, but the Service Provider only builds TVCs from the Headquarters site to each remote site (and not between the remote sites) then the SD-WAN Service is, effectively, a hub and spoke even though the Underlay Connectivity Service provides a full mesh.



Study Requirement

MEF-SDCP Exam Study Reference

Project Lead

1 Comment

  1. Regarding Diagram above: The position of the 'dot' at the end of TVC line closer to the UCS UNI than the SD-WAN UNI has lead to some confusion within the training material. Encountered training material that assumed "hairpin switching" could occur between the termination points of TCS into an SD-WAN Edge, without traversing the SD-WAN UNI. I'm uncertain if this was intended behavior of the authors of MEF70; the document is silent on this, with one example of using another site's 'internet breakout' as out of scope. This wiki could clear up the confusion while we await MEF70.1 content to clear this up. R13 appears to be the closest constraint to help explain this, but unfortunately Internet Breakout isn't an SWVC, so it doesn't exactly apply.