# MPLS Interfaces and Interface Stacking Overview

The JunosE implementation of MPLS employs MPLS major, minor, and shim interfaces.

## MPLS Major Interfaces

An MPLS major interface must be stacked on a layer 2 interface to send or receive MPLS packets on that interface. Each MPLS major interface exists in a particular virtual router.

MPLS major interfaces can use the platform label space or the interface label space. Which type of label space is used by the major interface is determined by the layer 2 interface on which the major interface is stacked. If the layer 2 interface is an ATM AAL5 interface, the major interface uses the interface label space. For all other layer 2 interface types, the major interface uses the platform label space.

When an MPLS packet arrives on the MPLS major interface, MPLS looks up the label of the received MPLS packet, the in label, in the MPLS forwarding table that is associated with the major interface. For major interfaces using the platform label space, the lookup is in the MPLS forwarding table of the VR. For major interfaces using the interface label space, the lookup is in the MPLS forwarding table of the major interface.

You use the mpls command in Interface Configuration mode to create or remove MPLS major interfaces. Some other commands create an MPLS major interface if it does not already exist.

You can configure the following attributes for each MPLS major interface:

- The administrative state, enabled or disabled, configured with the mpls disable command.
- The range of ATM VPIs used as interface labels for major interfaces stacked on ATM AAL5 interfaces, configured with the mpls atm vpi range command.
- The range of ATM VCIs used as interface labels for major interfaces stacked on ATM AAL5 interfaces, configured with the mpls atm vci range command.

## MPLS Minor Interfaces

When you configure an LSP with the interface tunnel mpls command, RSVP-TE creates an MPLS minor interface to represent the head of the LSP. MPLS minor interfaces are also created by RSVP-TE on the transit and tail LSRs when the LSP is signaled. Only RSVP-TE creates MPLS minor interfaces. Neither BGP nor LDP create them.

These minor interfaces are used to associate policy or a QoS profile with an LSP (either on an LSR or an LER). This minor interface is created automatically by the signaling protocol. Minor interfaces are not saved in NVS. Use the show mpls interface minor command to view the minor interfaces.

The following attributes of the minor interface are set by RSVP-TE:

- The UID of the minor interface, assigned automatically when the interface is created.
- The operational state of the interface, up or down.
- Whether the interface is an ingress MPLS minor interface used to receive traffic or an egress MPLS minor interface used to transmit traffic.

## MPLS Shim Interfaces

MPLS shim interfaces are stacked on layer 2 interfaces to provide layer 2 services over MPLS or to create local cross-connects by cross-connecting the layer 2 interface to another layer 2 interface. For more information about MPLS shim interfaces, see Configuring Layer 2 Services over MPLS.

## Interface Stacking

MPLS interface stacking differs depending on whether the platform label space (Figure 56) or the interface label space (Figure 57) is used.

Figure 56: MPLS Interface Stacking for the Platform Label Space

Figure 57: MPLS Interface Stacking for the Interface Label Space

#### Related Documentation

- MPLS Label Distribution Methodology
- MPLS Label Switching and Packet Forwarding Overview
- MPLS Label Distribution Protocols Overview