What are the different types of MPLS labels and how are they used?

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MPLS, or Multiprotocol Label Switching, is a technique that enables fast and efficient forwarding of network traffic based on labels. Labels are short identifiers that are attached to packets by MPLS-enabled routers, or label switch routers (LSRs), and used to make routing decisions without inspecting the packet contents. In this article, you will learn about the different types of MPLS labels and how they are used in various scenarios.

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1 Label format

An MPLS label consists of four fields: a 20-bit label value, a 3-bit experimental (EXP) field for quality of service (QoS) and traffic engineering (TE) purposes, a 1-bit bottom of stack (S) flag that indicates whether the label is the last one in a label stack, and an 8-bit time to live (TTL) field that prevents packets from looping indefinitely. The label value is the most significant part of the label, as it determines the next hop and the action to be taken on the packet. This value can range from 0 to 1,048,575, but some are reserved for specific purposes. The format of the label is illustrated below:

0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Label                  | Exp |S|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Understanding this format is essential in order to effectively use MPLS labels.

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Cristian Critelli

EMEA Lead Networking & Resilience Specialist Partner Solution Architect at Amazon Web Services (AWS) [ex Microsoft Azure]
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Ok let’s list them below ?? MPLS uses 32-bit labels with: 1. Label Value (20 bits): For forwarding decisions. 2. Traffic Class (3 bits): QoS prioritization. 3. Bottom of Stack (1 bit): Indicates the last label. 4. Time to Live (8 bits): Prevents looping. Labels are used for: - LSP Labels: Guide packets in MPLS networks. - VPN Labels: Separate data in MPLS VPNs. - Traffic Engineering Labels: Manage network bandwidth and congestion. - QoS Labels: Ensure traffic meets bandwidth and latency requirements. Assigned at network ingress, these labels are switched by routers within MPLS networks, optimizing efficiency for diverse networking needs.

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Desmond Durrant

PMP | CISSP | CCSE | CISO | CCIE | MCSA | MCSE AZ 300 | CKA | AZ 500 | AZ 301 | AZ 900 | AZ104
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Multiprotocol Label Switching (MPLS). MPLS is a routing technique used in telecommunications networks to direct data from one node to the next based on labels rather than traditional network addresses. Here’s a breakdown of the key components of an MPLS label: Label (20 bits): The label value itself, spanning 20 bits, uniquely identifies a specific label. It serves as a crucial identifier for established paths between network endpoints. Instead of examining the entire packet, MPLS decisions are made solely based on this label content. Experimental (EXP) Field (3 bits): The EXP field is an extension field within the MPLS label. Typically used for Quality of Service (QoS) purposes, it allows for differentiating traffic classes.

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2 Reserved labels

MPLS has five reserved label values that each have a predefined meaning and function. Label 0 is the IPv4 Explicit Null label and it indicates that the packet should be forwarded as an IPv4 packet with the label removed at the egress LSR. Label 1, the Router Alert label, is used to signal that the packet contains information requiring attention from the LSR, such as an OAM or RSVP message. Label 2 is the IPv6 Explicit Null label, indicating that the packet should be forwarded as an IPv6 packet and the label removed before reaching the egress LSR. Label 3, known as Implicit Null or penultimate hop popping (PHP), is used by the ingress LSR to inform the penultimate LSR to remove the label and forward the packet based on its IP header. Finally, Label 4 is Reserved for future use and should not be assigned to any packet.

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3 Label assignment

There are two main methods for assigning labels to packets in MPLS: downstream unsolicited (DU) and downstream on demand (DoD). In the DU method, an LSR assigns a label to a destination prefix and advertises it to its upstream neighbors without waiting for a request. In the DoD method, an LSR assigns a label to a destination prefix only when it receives a request from its upstream neighbor. The request and the advertisement are done using a label distribution protocol (LDP), which is a protocol that enables LSRs to exchange label information and establish label switched paths (LSPs).

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Desmond Durrant

PMP | CISSP | CCSE | CISO | CCIE | MCSA | MCSE AZ 300 | CKA | AZ 500 | AZ 301 | AZ 900 | AZ104
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MPLS (Multiprotocol Label Switching) and explore how labels are assigned to packets. Downstream Unsolicited (DU): In this label distribution mode, an LSR (Label-Switched Router) advertises label mappings to its peers without being explicitly asked. Essentially, it proactively shares label binding information. When an LSR has made label bindings, it informs other LSRs of these bindings without waiting for a specific request. DU is useful for scenarios where hop-by-hop forwarding is required, such as in MPLS virtual private networks (VPNs). Downstream on Demand (DoD): In the DoD mode, an LSR provides label mappings to a peer only when that peer explicitly requests them. When an LSR receives a request from a downstream LSR on a label-switched.

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4 Label switching

Label switching is the process of forwarding packets based on their labels. An LSR performs label switching by looking up the incoming label in its label forwarding information base (LFIB), which is a table that maps labels to next hops and outgoing labels. The LSR then swaps the incoming label with the outgoing label, adjusts the EXP and TTL fields, and sends the packet to the next hop. The packet traverses the network along an LSP, which is a sequence of LSRs that share the same label information for a given destination. The packet reaches the egress LSR, which removes the label and forwards the packet based on its IP header.

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5 Label stacking

Label stacking is a technique that allows multiple labels to be attached to a packet, forming a label stack. The label stack is read from top to bottom, and each label corresponds to a different level of hierarchy or context. For example, a packet can have a label stack that consists of a VPN label, a TE label, and a LDP label, indicating that the packet belongs to a VPN service, follows a TE path, and uses LDP for label distribution. Label stacking enables MPLS to support various applications and features, such as VPNs, TE, pseudowires, and hierarchical LSPs.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the different types of MPLS labels and how are they used?

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3

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6

1 Label format

2 Reserved labels

3 Label assignment

4 Label switching

5 Label stacking

6 Here’s what else to consider

Network Engineering

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What are the different types of MPLS labels and how are they used?

1

2

3

4

5

6

1 Label format

2 Reserved labels

3 Label assignment

4 Label switching

5 Label stacking

6 Here’s what else to consider

Network Engineering

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