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Troubleshooting Virtual Private Networks by Mark Lewis

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Technical Overview of AToM

A good understanding of AToM is essential to allow fast and efficient troubleshooting. This section, therefore, contains an examination of the operation of AToM.

Note that it is a good idea to read the section entitled “MPLS Architecture” in Chapter 6, “Troubleshooting Multiprotocol Label Switching Layer 3 VPNs,” if you do not already have a good understanding of MPLS.

Layer 2 PDU Transport

Layer 2 PDUs are transported over the MPLS backbone between attachment circuits (circuits between PE and CE devices). This transportation occurs by prepending a control word, a VC label, and one or more tunnel labels (assuming PE routers are not in a back-to-back configuration) to the Layer 2 PDU itself. Figure 7-2 illustrates this ...

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