MPLS Header Structure Fields

H12-811 By islever Editorial Last updated

Short answer

ACD The MPLS header consists of the Label (20 bits), EXP (3 bits), S (1 bit), and TTL (8 bits) fields.

Question

Which of the following fields are included in an MPLS header? (Choose three.)
  1. Label Correct Answer
  2. Tos
  3. EXP Correct Answer
  4. TTL Correct Answer

Key Takeaways

Test your knowledge of the 32-bit MPLS label breakdown: 20-bit Label, 3-bit EXP, 1-bit S, and 8-bit TTL. Confuse MPLS EXP with IP ToS/TOS?

Identify the three fields present in an MPLS header: Label, EXP, and TTL. This guide explains the 32-bit MPLS label structure and distinguishes it from IP header fields like ToS.

Common mistake: Choosing 'ToS' (Type of Service) instead of 'EXP'. Both relate to QoS, but ToS is an IPv4 header field, while EXP is the specific MPLS equivalent for carrying traffic priority.

Expert Analysis

The question asks for the fields included in an MPLS header, requiring three correct options. The MPLS label is a fixed 32-bit structure inserted between the Layer 2 and Layer 3 headers.

Label (20 bits): This is the core identifier of the Forwarding Equivalence Class (FEC). It determines how the packet is forwarded along the Label Switched Path (LSP).

EXP (3 bits): Originally labeled as 'Experimental', these bits are used to carry Quality of Service (QoS) information, such as traffic priority or class-of-service markings, allowing MPLS networks to support differentiated services.

TTL (8 bits): The Time To Live field functions similarly to IP TTL. It is decremented at each hop to prevent routing loops and can be copied from the underlying IP header upon insertion.

The remaining bit is the S (1 bit) or Bottom-of-Stack indicator, which is not listed in the options.

Option B (Tos) is incorrect because Type of Service is a field within the IPv4 header, not the MPLS header. While EXP serves a similar QoS purpose, they are distinct fields in different protocol layers. Therefore, the three correct fields are Label, EXP, and TTL.

Official Reference

https://support.huawei.com/enterprise/en/doc/EDOC1100118961

This link opens the official vendor documentation in a new tab.

Exam Strategy

Memorize the 32-bit composition of the MPLS label: 20+3+1+8. When asked about header fields, distinguish between Layer 3 (IP) and Label layer attributes. If an option looks like an IP header field (like ToS or Source IP), it is likely incorrect for MPLS unless it refers to a specific encapsulation mapping.

Frequently Asked Questions

What does the S bit stand for in MPLS?

The S bit stands for Bottom-of-Stack. It indicates whether the current label is the last one in the stack.

Is EXP the same as ToS?

No. ToS is in the IP header. EXP is in the MPLS header and carries similar QoS priority information but is distinct.

More H12-811 FAQ →

Discussion 6 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 6

  1. buggybombastis10 👍 2
    The answeri is ACD Mpls header contain label, exp, bos, ttl
  2. hafizhou 👍 1
    The MPLS (Multiprotocol Label Switching) header includes the following fields: A. Label C. EXP (Experimental) D. TTL (Time To Live) Explanation: A. Label: The MPLS label is a fixed-size field that is used to identify the forwarding equivalence class (FEC) and to determine the next hop in the label-switched path (LSP). C. EXP (Experimental): EXP bits are used for experimental purposes, allowing the definition of Class of Service (CoS) values for QoS (Quality of Service). D. TTL (Time To Live): The TTL field is used to limit the time that a packet can circulate in the network. It is decremented at each hop, and if it reaches zero, the packet is discarded.
  3. Fmoura17 👍 1
    A. Label, C. EXP, D. TTL MPLS Header Structure (32 bits total): | Label (20 bits) | EXP (3 bits) | S (1 bit) | TTL (8 bits) | Field Descriptions: FieldBitsDescriptionLabel20 bitsIdentifies the MPLS path (LSP) used for forwardingEXP3 bitsExperimental bits, used for QoS/traffic priority markingS (Bottom of Stack)1 bitIndicates if this is the last label in the stack (S=1)TTL8 bitsTime To Live, prevents routing loops (same concept as IP TTL)
  4. NaCin 👍 1
    Here is a table representation of an MPLS header: Field Size (bits) Description Label 20 Used to identify the Forwarding Equivalence Class (FEC) for the packet. EXP 3 Experimental bits used for Quality of Service (QoS). S 1 Bottom of Stack bit. Set to 1 for the last label in the label stack. TTL 8 Time to Live. Decremented at each hop and used to prevent routing loops.
  5. surveycorps969 👍 1
    Check "MPLS Implementation" section. https://support.huawei.com/enterprise/en/doc/EDOC1100118961
Show 1 more comment
  1. Jrmelo 👍 1
    Answer is ACD

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