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MPLS Fundamentals by Luc De Ghein - CCIE No. 1897

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The Control Word

The control word is a 32-bit field that is inserted between the VC label and the transported Layer 2 frame in the case of AToM. It is required for some Layer 2 protocols but optional for others. The control word carries extra information such as protocol control information and a sequence number and does this in a compressed format. This information is needed to correctly and efficiently carry the Layer 2 protocol across the MPLS network. The ingress PE router adds the control word, and the egress PE router strips it off after processing it. Whether the control word is used can be signaled by the PE routers or can be configured. In both cases, the egress PE router knows that it should expect the presence of the control word ...

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