The previous two chapters showed that you can deploy MPLS using different modes of operation. Chapter 2, "Frame-mode MPLS Operation," details how MPLS operates across framed interfaces, and Chapter 3, "Cell-mode MPLS Operation," shows how MPLS operates natively across ATM media.
The Layer 2 infrastructure, which provides the media over which Frame-mode MPLS can operate, often can be supplied through the use of switched WAN technology, such as Frame Relay or ATM. You can run MPLS in Cell-mode across ATM but not when using Frame Relay or when the ATM structure is built using traditional ATM Forum PVCs. This means that it must be possible to run Frame-mode MPLS across these types of interfaces so you can deploy MPLS end-to-end across the network.
This chapter considers the deployment of MPLS across Frame Relay interfaces and ATM PVCs. It also considers the use of Frame-mode and Cell-mode MPLS across the same physical interface; this functionality can be useful during a migration to the MPLS architecture, as you'll learn in Chapter 6, "MPLS Migration and Configuration Example."