Assume that there are 5 Segment IDs in the SRH of an SRv6 message. When the message reaches the third End Point node, which of the following values does the SL field in the SRH take?

Question

Options

• A1
• B2
• C3
• D5

• A
  12% (3)
• B
  77% (20)
• C
  4% (1)
• D
  8% (2)

Explanation

Option B (SL = 2) is correct because in SRv6, the Segments Left (SL) field is initialized to N−1 (one less than the total number of SIDs), and each endpoint node decrements SL by 1 upon processing the packet. With 5 SIDs, SL starts at 4, drops to 3 after the 1st endpoint, to 2 after the 2nd - meaning when the packet arrives at the 3rd endpoint, SL = 2.

Why the distractors are wrong:

• A (1): SL = 1 occurs when the packet arrives at the 4th endpoint, not the 3rd.
• C (3): SL = 3 is the value after the 1st endpoint processes the packet (i.e., when the packet is en route to the 2nd endpoint).
• D (5): SL can never equal 5 here - it starts at N−1 = 4 and only decrements, so 5 is an impossible value.

Memory tip: Use the formula SL = N − n, where N is the total number of SIDs and n is the endpoint number the packet is arriving at. Here: 5 − 3 = 2.

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