Assume that there are 5 Segment IDs in the SRH of an SRv6 message. When the message is sent from the third End Point, the node will set the Segment ID numbered as which of the following to the IPv6…

Question

Options

• A5
• B1
• C3
• D2

• A
  4% (1)
• B
  74% (20)
• C
  15% (4)
• D
  7% (2)

Explanation

Option B is correct because SRv6's Segment Routing Header stores SIDs in reverse traversal order, indexed from 0. With 5 SIDs, the Segments Left field starts at 4 (pointing to SID[4], the first hop). Each endpoint decrements Segments Left by 1 and updates the IPv6 destination address to Segment List[new Segments Left]. After the 3rd endpoint processes its segment, Segments Left drops from 2 to 1, so the node sets the IPv6 DA to Segment List[1] - Segment ID number 1.

• A (5) is wrong - 5 is the total SID count, not a valid Segments Left index (indices run 0–4).
• C (3) is wrong - SID[3] becomes the destination after the 1st endpoint, not the 3rd.
• D (2) is wrong - SID[2] is set after the 2nd endpoint; it's an off-by-one error that conflates the hop count with the resulting index.

Memory tip: Use the formula Segments Left after nth hop = (n − 1) − n_hops. With 5 SIDs: after hop 3 → 4 − 3 = 1. Whatever that result is, that's the Segment ID number that becomes the next IPv6 destination.

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