In this chapter, you saw how to build Layer 2 virtual private networks (VPNs) over an MPLS infrastructure. You can build these VPNs by constructing pseudowires between PE routers that carry the Layer 2 frames from the attachment circuits (ACs) that are attached to the PE router. ACs can have a variety of encapsulation types, including High-Level Data Link Control (HDLC), PPP, Frame Relay, ATM, and Ethernet. Frame Relay can be transported in DLCI-to-DLCI mode or in port-to-port mode. ATM can be transported in AAL5, single cell relay, or packed cell relay mode. Finally, you saw some specific Any Transport over MPLS (AToM) issues related to maximum transmission unit (MTU), quality of service (QoS), and AToM Tunnel Selection.