MPLS S Field Bottom of Stack Logic

H12-811 By islever Editorial Last updated

IP Routing Basics

Short answer

CD The S field uses 0 to indicate more labels follow and 1 to indicate the bottom of the stack, and this 1-bit field is present in every MPLS label header.

Question

Which of the following statements about the "S" field in the MPLS packet header are true? (Choose all that apply.)
  1. The field indicates whether there are other labels behind the current label. 1: Yes; 0: No.
  2. The "S' field is 1 bit long in frame mode and 2 bits long in cell mode.
  3. The field indicates whether there are other labels behind the current label. 0: Yes; 1: No. Correct Answer
  4. The "S" field exists in each MPLS packet header. Correct Answer

Key Takeaways

The main trap is reversing the S bit logic (Option A); remember that S=1 means 'Stop/Bottom' (no more labels), while S=0 means 'Continue' (more labels below).

This question tests knowledge of the MPLS label header's S (Bottom of Stack) field, confirming that it uses specific bit values to denote stack depth and exists in every label entry.

Common mistake: Candidates often select Option A because they know the S field indicates the end of the stack but mistakenly associate '1' with 'Yes, there are more labels'. In reality, S=1 means 'No, this is the bottom'.

Expert Analysis

The MPLS label header is a fixed 32-bit structure consisting of four fields: the Label (20 bits), Exp/BotS (3 bits), TTL (8 bits), and S (1 bit). The 'S' field, often called the Bottom of Stack (BoS) bit, serves as a flag to indicate whether the current label is the last one in the stack.

Option A is incorrect because it reverses the logic. An 'S' value of 1 signifies that there are no other labels behind the current label (it is the bottom). An 'S' value of 0 indicates that more labels follow. Option B is incorrect because the 'S' field is strictly 1 bit long in both frame mode and cell mode; its length does not vary by encapsulation type.

Option C correctly defines the bit logic: 0 means yes (more labels exist), and 1 means no (this is the bottom). Option D is also correct because every individual label in an MPLS stack possesses its own header containing an 'S' bit, allowing each layer to be identified independently. Therefore, the 'S' field exists in each MPLS packet header component.

The most attractive wrong answer is A. It sounds plausible because it describes the function of the field but gets the boolean mapping reversed—a common trap in networking exams where students memorize '1 = end' but forget how '0/1' maps to 'yes/no' questions.

Exam Strategy

When analyzing header fields, always verify the binary value-to-meaning mapping carefully. For flags like MPLS S or TCP ACK, determine if a '1' represents an active state or a termination state. Draw a quick diagram of the label stack to visualize how the S bit at each level points to the next or terminates the sequence.

Frequently Asked Questions

What does the MPLS S field stand for?

The S field stands for 'Bottom of Stack' (BoS). It is a 1-bit flag in the MPLS label header.

Is the MPLS S field size different in ATM cells vs Ethernet frames?

No. The S field is always 1 bit long regardless of whether the MPLS packet is carried over frame-mode networks (like Ethernet) or cell-mode networks (like ATM).

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Community Votes

D
50%
C
25%
A
25%

50% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 7 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 7

  1. 4c4da48 👍 2
    A. "The field indicates whether there are other labels behind the current label. 1: Yes; 0: No." — This is incorrect. It's the opposite. B. "The 'S' field is 1 bit long in frame mode and 2 bits long in cell mode." — This is false. The 'S' field is always 1 bit long. C. "The field indicates whether there are other labels behind the current label. 0: Yes; 1: No." — This is true. D. "The 'S' field exists in each MPLS packet header." — This is true.
  2. iitc_duo 👍 2
    C and D, I have checked in AI (Chat GPT and Gemini)
  3. hafizhou 👍 2
    AD CORRECT
  4. branly 👍 1
    El campo "S", también conocido como el bit de pila, tiene una longitud de 1 bit y se utiliza para indicar si la etiqueta actual es la última en la pila (BoS). Si el bit "S" está configurado en 1, significa que la etiqueta actual es la última en la pila y no hay otras etiquetas detrás de ella. Si el bit "S" está configurado en 0, significa que hay más etiquetas en la pila detrás de la etiqueta actual. Este campo está presente en cada encabezado de paquete MPLS para gestionar eficazmente la pila de etiquetas La afirmación B es incorrecta porque el campo "S" siempre tiene una longitud de 1 bit, independientemente de si el modo es marco o celda. La afirmación C también es incorrecta porque describe inversamente la función del campo "S"
  5. Fmoura17 👍 1
    Answer: C and D C. ✅ — The S (Bottom of Stack) bit indicates: S = 1 → This is the last label (bottom of stack), no more labels below. S = 0 → There are more labels below this one. D. ✅ — The S field exists in every MPLS label header (each label in the stack has its own S bit). Why A and B are wrong: A. ❌ — The logic is reversed. S=1 means no more labels (bottom of stack), not that there are more labels. B. ❌ — The S field is always 1 bit long regardless of frame or cell mode.
Show 2 more comments
  1. foetus 👍 1
    Incorrect. For A it is exactly as described. S label set to 1 means there are no more labels behind it. So the sentence is correct. C & D are correct.
  2. foetus 👍 1
    A&D* (not C&D)

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