300-410 Infrastructure Services Practice Question
A network engineer is configuring a Cisco IOS XE router to support MPLS L3VPN. The router is a PE device with a VRF named CUSTOMER. The engineer wants to ensure that the PE router can forward traffic for the CUSTOMER VRF using MPLS labels. Which command must be configured on the PE router's core-facing interface to enable MPLS forwarding?
⚠ Common exam trap
Watch out — candidates often confuse commands that configure MPLS label distribution (like mpls label protocol ldp) with the command that actually enables MPLS forwarding on an interface (mpls ip).
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
mpls ip
To enable MPLS forwarding on a Cisco IOS XE router interface, the mpls ip command must be configured. This command instructs the interface to process MPLS labels and forward labeled packets. On a PE router, the core-facing interface must have mpls ip enabled to send and receive MPLS traffic for VPNs. While other MPLS-related commands configure label distribution or router IDs, they do not enable the actual forwarding of MPLS packets on the interface.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
mpls label protocol ldp
Why it's wrong here
The mpls label protocol ldp command specifies that LDP should be used as the label distribution protocol on the interface. While it is often configured along with mpls ip, it does not itself enable MPLS forwarding. It only sets the protocol for label exchange. The interface still requires mpls ip to actually forward MPLS packets. This command alone is insufficient.
- ✗
mpls ldp router-id Loopback0
Why it's wrong here
The mpls ldp router-id Loopback0 command sets the LDP router ID to the IP address of Loopback0. This is a global configuration command that influences LDP session establishment and label distribution, but it does not enable MPLS forwarding on any interface. It is a separate configuration step and does not directly affect the interface's ability to forward labeled packets.
- ✗
mpls ldp discovery transport-address interface
Why it's wrong here
The mpls ldp discovery transport-address interface command configures LDP to use the interface's IP address as the transport address for LDP sessions. It is used in certain LDP configuration scenarios, such as when you want LDP to use a specific address for TCP sessions. It does not enable MPLS forwarding on the interface; that requires the mpls ip command.
- ✓
mpls ip
Why this is correct
The mpls ip command enables MPLS forwarding on an interface. On the core-facing interface of a PE router, this command is required to allow the interface to send and receive MPLS-labeled packets. Without it, the interface will not process MPLS labels, and traffic for the CUSTOMER VRF will not be forwarded correctly across the MPLS backbone.
Go deeper
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Key term
MPLS Layer 3 VPN
A technology that uses Multiprotocol Label Switching to create secure, scalable virtual private networks that connect multiple sites at the network layer, where the service provider manages routing between customer sites.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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