Comprehensive and Detailed In-Depth Explanation:
1. Understanding MPLS Label Stack and the S (Bottom of Stack) Bit
MPLS (Multiprotocol Label Switching) packets can have multiple labels stacked on top of each other.
Each label has the following fields:
Label (20 bits): Identifies the forwarding path.
EXP (3 bits): Used for QoS/priority marking.
TTL (8 bits): Time-to-live, similar to IP TTL.
S (1 bit): Bottom of Stack (BoS) indicator:
S = 0: This is not the last label in the stack.
S = 1: This is the last label in the stack, meaning the next header is the IP header.
2. Analyzing the MPLS Packet Structure in the Figure
The diagram shows three MPLS headers (labels).
The last MPLS header before the IP header must have S = 1, indicating the bottom of the stack.
All other MPLS headers above it must have S = 0, meaning they are part of the stacked labels.
3. Evaluating the Answer Choices
Option
S1
S2
S3
Correct?
Reasoning
A
3
2
1
❌ Incorrect
S field cannot have values greater than 1.
B
2
1
0
❌ Incorrect
S field must be 0 for non-bottom labels.
C
1
1
0
❌ Incorrect
There should be only one label with S = 1 (BoS).
D
0
0
1
✅ Correct
Only the last label should have S = 1.
✅ Correct Answer: D. S1 = 0, S2 = 0, S3 = 1
Final Conclusion:
Only the last MPLS label (closest to the IP header) should have S = 1.
All previous MPLS labels should have S = 0.
Thus, the correct answer is: D. S1 = 0, S2 = 0, S3 = 1
[Reference:, HCIP-Datacom-Advanced Routing & Switching Technology V1.0 – MPLS Label Stack and Header Format, Huawei Official HCIP-Datacom Study Guide – MPLS Label Encoding and Forwarding, Huawei Documentation on MPLS Label Stack and Bottom of Stack (BoS) Bit, , , ]