In this chapter, you see how you can run MPLS across traditional WAN media, be it a Frame Relay network or an ATM network supporting only ATM Forum permanent VCs. In both cases, MPLS runs directly between routers connected to the WAN network and the WAN switches are not aware of MPLS being transported across the WAN network.
MPLS over a Frame Relay network or an ATM Forum PVC is configured as Frame-mode MPLS and the LDP/TDP session is established directly between routers connected to the WAN network. The routers use standard Frame-mode label allocation and distribution procedures and forward labeled packets as frames with the standard label header (like any other datagrams, these frames obviously are transported as cells across the ATM network).
There are several benefits gained by running MPLS in Frame-mode across a WAN network:
You can use the existing WAN infrastructure for transporting labeled packets.
You can run MPLS across ATM networks that are not yet MPLS-enabled or across public ATM networks that do not support MPLS services.
The migration of ATM networks toward MPLS becomes simple as the Frame-mode MPLS between routers offers a very convenient first transitional step.
The drawback of running MPLS across a WAN network in Frame-mode is also obvious?you're again faced with the scalability issues of the large WAN networks discussed in Chapter 1, "Multiprotocol Label Switching (MPLS) Architecture Overview."