Cisco Cisco IOS Software Release 12.0(22)S

Each routed PVC label stack has two levels of labels prepended to each ATM SDU: 

An Interior Gateway Protocol (IGP) stack consisting of zero or more labels

A PVC-based label

Routers at the edge of the MPLS backbone perform label imposition and disposition. The imposition 
router encapsulates the ATM SDU into an MPLS SDU to transport it to the correct disposition router. 
The disposition router takes the MPLS SDU, de-encapsulates the ATM SDU, and delivers it to the 
correct ATM interface and virtual path identifier/virtual circuit identifier (VPI/VCI).

As Internet service providers (ISPs) begin to deploy IP/MPLS backbones, services including ATM frame 
switching must be supported. The AAL5  over MPLS feature allows an ISP to transport AAL5 frames 
over an IP/MPLS backbone consisting of Cisco 12000 Series router platforms with Packet over SONET 
(POS) links.

The following restriction apply to AAL5 over MPLS:

The AAL5 over MPLS feature supports only AAL5 SDU switching. 

The following services are not supported with the AAL5 over MPLS feature:

ATM switched virtual circuits (SVCs)

ATM signaling

Fragmentation and reassembly

Traffic shaping of packets in the MPLS-to-ATM path

The instructions for configuring AAL5 over MPLS in this feature module apply only to the Cisco 
12000 series router for 12.0(21)ST. Avoid using these instructions for any other platform or release. 

Refer to the following documents for more information: