The Core Network
On the core side, after an ATM cell has gone through the segment assembly and reassembly (SAR) process, and Layer 3 (IP) information is retrieved, the access concentrator must decide what to do with that packet. If the access concentrator belongs to the NAP, it routes that packet according to the destination of the packet. However, if this access concentrator is only a transit point for other NSPs, the access concentrator forwards that packet to the appropriate SPs by using one of the following methods:
ATM
IP
MPLS
L2TP
The sections that follow discuss the methods not covered so far—L2TP and MPLS.
As discussed in the sections on access architectures, if the core is ATM-based and if the access concentrator in the NAP is terminating ...
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