Which list correctly defines the fields of an MPLS label?
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Correct answer: 20-bit label; 3-bit traffic class; 1-bit bottom stack; 8-bit TTL.
Why this is the answer
The MPLS label header is 32 bits long and consists of four fields. The 'label' field is 20 bits, used for forwarding decisions. The 'traffic class' (TC) field, formerly known as Experimental (EXP), is 3 bits and is used for QoS marking and ECN. The 'bottom of stack' (S) field is 1 bit, indicating if this is the last label in the stack. The 'Time to Live' (TTL) field is 8 bits, preventing loops and decrementing at each hop, similar to IP TTL. The incorrect options either misstate the label length (32-bit label is wrong, as the entire header is 32 bits, not just the label field), use incorrect terminology like 'flow label' or 'hop limit' (which are IP-related terms, not MPLS), or combine these errors.
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