Multiprotocol Label Switching
This module describes Multiprotocol Label Switching and how to configure it on Cisco switches.
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This module describes Multiprotocol Label Switching and how to configure it on Cisco switches.
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Multiprotocol Label Switching (MPLS) fragmentation is not supported.
MPLS maximum transmission unit (MTU) is not supported.
Multiprotocol label switching (MPLS) combines the performance and capabilities of Layer 2 (data link layer) switching with the proven scalability of Layer 3 (network layer) routing. MPLS enables you to meet the challenges of explosive growth in network utilization while providing the opportunity to differentiate services without sacrificing the existing network infrastructure. The MPLS architecture is flexible and can be employed in any combination of Layer 2 technologies. MPLS support is offered for all Layer 3 protocols, and scaling is possible well beyond that typically offered in today’s networks.
Label switching is a high-performance packet forwarding technology that integrates the performance and traffic management capabilities of data link layer (Layer 2) switching with the scalability, flexibility, and performance of network layer (Layer 3) routing.
In conventional Layer 3 forwarding mechanisms, as a packet traverses the network, each switch extracts all the information relevant to forwarding the packet from the Layer 3 header. This information is then used as an index for a routing table lookup to determine the next hop for the packet.
In the most common case, the only relevant field in the header is the destination address field, but in some cases, other header fields might also be relevant. As a result, the header analysis must be done independently at each switch through which the packet passes. In addition, a complicated table lookup must also be done at each switch.
In label switching, the analysis of the Layer 3 header is done only once. The Layer 3 header is then mapped into a fixed length, unstructured value called a label .
Many different headers can map to the same label, as long as those headers always result in the same choice of next hop. In effect, a label represents a forwarding equivalence class --that is, a set of packets which, however different they may be, are indistinguishable by the forwarding function.
The initial choice of a label need not be based exclusively on the contents of the Layer 3 packet header; for example, forwarding decisions at subsequent hops can also be based on routing policy.
Once a label is assigned, a short label header is added at the front of the Layer 3 packet. This header is carried across the network as part of the packet. At subsequent hops through each MPLS switch in the network, labels are swapped and forwarding decisions are made by means of MPLS forwarding table lookup for the label carried in the packet header. Hence, the packet header does not need to be reevaluated during packet transit through the network. Because the label is of fixed length and unstructured, the MPLS forwarding table lookup process is both straightforward and fast.
Each label switching router (LSR) in the network makes an independent, local decision as to which label value to use to represent a forwarding equivalence class. This association is known as a label binding. Each LSR informs its neighbors of the label bindings it has made. This awareness of label bindings by neighboring switches is facilitated by the following protocols:
Label Distribution Protocol (LDP)--enables peer LSRs in an MPLS network to exchange label binding information for supporting hop-by-hop forwarding in an MPLS network
Border Gateway Protocol (BGP)--Used to support MPLS virtual private networks (VPNs)
When a labeled packet is being sent from LSR A to the neighboring LSR B, the label value carried by the IP packet is the label value that LSR B assigned to represent the forwarding equivalence class of the packet. Thus, the label value changes as the IP packet traverses the network.
For more information about LDP configuration, see the see MPLS: LDP Configuration Guide at http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mpls/config_library/xe-3s/mp-xe-3s-library.html
![]() Note |
As the scale of label entries is limited in, especially with ECMP, it is recommended to enable LDP label filtering. LDP labels shall be allocated only for well known prefixes like loopback interfaces of routers and any prefix that needs to be reachable in the global routing table. |
A Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) consists of a set of sites that are interconnected by means of an MPLS provider core network. At each customer site, one or more customer edge (CE) routers attach to one or more provider edge (PE) routers.
Before configuring MPLS Layer 3 VPNs, you should have MPLS, Label Distribution Protocol (LDP), and Cisco Express Forwarding (CEF) installed in your network. All routers in the core, including the PE routers, must be able to support CEF and MPLS forwarding.
The QoS EXP Matching feature allows you to classify and mark network traffic by modifying the Multiprotocol Label Switching (MPLS) experimental bits (EXP) field in IP packets.
The QoS EXP Matching feature allows you to organize network traffic by setting values for the MPLS EXP field in MPLS packets. By choosing different values for the MPLS EXP field, you can mark packets so that packets have the priority that they require during periods of congestion. Setting the MPLS EXP value allows you to:
Classify traffic: The classification process selects the traffic to be marked. Classification accomplishes this by partitioning traffic into multiple priority levels, or classes of service. Traffic classification is the primary component of class-based QoS provisioning.
Police and mark traffic: Policing causes traffic that exceeds the configured rate to be discarded or marked to a different drop level. Marking traffic is a way to identify packet flows to differentiate them. Packet marking allows you to partition your network into multiple priority levels or classes of service.
Following is the list of restrictions for classifying and marking MPLS QoS EXP:
Only Uniform mode and Pipe mode are supported; Short-pipe mode is not supported.
Support range of QoS-group values range between 0 and 30. (Total 31 QoS-groups).
EXP marking using QoS policy is supported only on the outer label; inner EXP marking is not supported.
This section explains how to perform the basic configuration required to prepare a switch for MPLS switching and forwarding.
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ip unnumbered command is not supported in MPLS configuration. |
Command or Action | Purpose | |
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Step 1 |
enable Example:
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Enables privileged EXEC mode. Enter your password, if prompted. |
Step 2 |
configure terminal Example:
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Enters global configuration mode. |
Step 3 |
ip cef distributed Example:
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Enables Cisco Express Forwarding on the switch. |
Step 4 |
mpls label range minimum-value maximum-value Example:
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Configure the range of local labels available for use with MPLS applications on packet interfaces. |
Step 5 |
mpls label protocol ldp Example:
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Specifies the label distribution protocol for the platform. |
![]() Note |
ip unnumbered command is not supported in MPLS configuration. |
Command or Action | Purpose | |||
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Step 1 |
enable Example:
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Enables privileged EXEC mode. Enter your password, if prompted. |
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Step 2 |
configure terminal Example:
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Enters global configuration mode. |
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Step 3 |
interface type slot / subslot / port Example:
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Step 4 |
mpls ip Example:
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Enables MPLS forwarding of IPv4 packets along routed physical interfaces (Gigabit Ethernet), Switch Virtual Interface (SVI), or port channels. |
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Step 5 |
mpls label protocol ldp Example:
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Specifies the label distribution protocol for an interface.
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Step 6 |
end Example:
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Exits interface configuration mode and returns to privileged EXEC mode. |
This section explains how to verify successful configuration of MPLS switching and forwarding.
To verify that Cisco Express Forwarding has been configured properly, issue the show ip cef summary command, which generates output similar to that shown below:
show ip cef summary Example:
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To verify that MPLS forwarding has been configured properly, issue the show mpls interfaces detail command, which generates output similar to that shown below:
![]() Note |
The MPLS MTU value is equivalent to the IP MTU value of the port or switch by default. MTU configuration for MPLS is not supported. |
Step 1 |
show mpls interfaces detail Example:
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Step 2 |
show running-config interface Example:
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Step 3 |
show mpls forwarding Example:
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Related Topic |
Document Title |
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For complete syntax and usage information for the commands used in this chapter. |
See the Multiprotocol Label Switching (MPLS) Commands section of the Command Reference (Catalyst 9300 Series Switches) |
Description |
Link |
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The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Release |
Modification |
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Cisco IOS XE Everest 16.5.1a |
This feature was introduced. |