Chapter 3. Cell-mode MPLS Operation
In Chapter 2, "Frame-mode MPLS Operation," you saw how MPLS is run between Layer 3 switching devices (routers) across Frame-mode interfaces. The routers running MPLS exchange pure IP packets (for control protocols) as well as labeled IP packets (forwarded or non-adjacent locally sourced traffic) over the same link. They also perform label switching by examining the label header in front of the IP packet.
When trying to fit the MPLS architecture into the limitations of ATM technology, a number of obstacles must be overcome:
There is no mechanism for direct exchange of IP packets between two adjacent MPLS nodes over an ATM interface. All data exchange over an ATM interface must take place over an ATM virtual circuit ...
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