From the get-go, the IP and ATM worlds seemed to clash. While ATM was being standardized, it envisioned IP coexisting with it, but always as a sideshow. Ever since the industry realized that we are not going to have our PCs and wristwatches running an ATM stack and that IP was here to stay, attempts have been made to map IP onto ATM. However, the main drawback of previous attempts to create a mapping between IP and ATM was that they either tried to keep the two worlds separate (carrying IP over ATM VCs) or tried to integrate IP and ATM with mapping services (such as ATM Address Resolution Protocol [ARP] and Next-Hop Resolution Protocol [NHRP]). Carrying IP over ATM VCs (often called the overlay model) is useful, but it has scalability limits; using mapping servers introduces more points of failure into the network.
The problem with the overlay approach is that it leads to suboptimal routing unless a full mesh of VCs is used. However, a full mesh of VCs can create many routing adjacencies, leading to routing scalability issues. Moreover, independent QoS models need to be set up for IP and for ATM, and they are difficult to match.
MPLS bridges the gap between IP and ATM. ATM switches dynamically assign virtual path identifier/virtual channel identifier (VPI/VCI) values that are used as labels for cells. This solution resolves the overlay-scaling problem without the need for centralized ATM-IP resolution servers. This is called Label-Controlled ATM (LC-ATM). Sometimes it is called IP+ATM.
For further details on ATM's role in MPLS networks, read the section "ATM in Frame Mode and Cell Mode" in Chapter 2.
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