In previous topics we have discussed about how:

• How MPLS is run between Layer 3 Switching devices across Frame-Mode interfaces.
• These routers exchange pure IP packets for control protocols as well as labeled IP Packets over the same link.
• They also performs label switching by examining the label header in front of the IP Packet.

When trying to fit MPLS architecture into the limitation of ATM technology, a number of below obstacles must be overcome:

• There is no mechanism for direct exchange of IP Packets between two adjacent MPLS nodes over ATM Interface, all the data exchange must be occur over the ATM interface by the Virtual Circuit (VC)
• ATM Switches doesn’t perform Label lookup or Layer 3 Lookup, the only capability of the ATM switch is to map the incoming VC in cell for the outgoing interface.

The ATM technology design and architecture present a number of challenges to an ATM implementation of MPLS Technologies as below:

• Control-Plane IP Packets can’t be exchanged directly over the ATM interfaces, a Control VC must be established between the MPLS enabled nodes to exchange control-plane packets.
• ATM don’t perform label lookup a top label must be translated to the VPI ( Virtual Path identifier and VCI Virtual Circuit Identifier.
• ATM don’t perform Layer 3 lookup, the label allocation and distribution procedures must be modified to makes sure that an ATM switch will never have to perform Layer 3 lookup.

Below are the Terms of ATM world for the ATM implementation of MPLS technology:

• LC-ATM ( Label Switching Controlled ATM Interface): This is an Interface on the ATM switch in which the VPI/VCI value is assigned through MPLS Control Protocol (LDP/TDP)
• ATM-LSR: This is a router that runs MPLS Protocols on the Control Plane and perform MPLS forwarding between LC-ATM interfaces on the data plane by tradition ATM Cell Switching
• Frame-Based LSR: Is an LSR that forwards complete frame between the interfaces, it can have a number of LC-ATM interfaces but it can only perform frame-based label switching base on label stack, not cell switching like an ATM-LSR
• ATM-LSR Domain: A set  of ATM-LSRs interconnected by LC-ATM Interfaces
• ATM Edge-LSR: Is a Frame-Based LSR with at least one LC-ATM interface

Cheers,