300-410 Infrastructure Services Practice Question
A network engineer is deploying MPLS Layer 3 VPNs on Cisco IOS routers. The engineer must ensure that customer routes are properly propagated across the MPLS core and that labels are correctly assigned. Which two protocols are used within the MPLS core to distribute labels and VPNv4 routes? (Choose two.)
⚠ Common exam trap
The trap here is assuming that any routing protocol that distributes labels, such as RSVP or BGP-LU, is used in MPLS L3VPNs, when in fact LDP and MP-BGP are the standard protocols for label and VPN route distribution.
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
✓
LDP
In an MPLS L3VPN, LDP is used to distribute labels for core routing, creating LSPs between PE routers. MP-BGP is used to distribute VPNv4 routes and their associated VPN labels between PE routers. Together, they enable the forwarding of customer traffic across the MPLS core with proper VPN isolation. OSPF, RSVP, and BGP-LU are not the standard protocols for these specific functions.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
LDP
Why this is correct
LDP (Label Distribution Protocol) is used to distribute labels for internal routes within the MPLS core. It maps IP prefixes to labels, enabling label switching for core routing. In an MPLS L3VPN, LDP is responsible for establishing label-switched paths (LSPs) between PE routers, which is essential for forwarding VPN traffic across the core.
- ✗
RSVP
Why it's wrong here
RSVP (Resource Reservation Protocol) is used for traffic engineering (RSVP-TE) to establish label-switched paths with bandwidth guarantees. It is not the default protocol for label distribution in an MPLS L3VPN; LDP is typically used. RSVP can distribute labels, but it is not a requirement for basic MPLS L3VPN operation, and it does not distribute VPNv4 routes.
- ✓
MP-BGP
Why this is correct
MP-BGP (Multiprotocol BGP) is used to distribute VPNv4 routes between PE routers. It carries the VPN label and route target information, allowing PEs to import and export routes into the correct VRFs. MP-BGP is the control plane protocol for MPLS L3VPNs, ensuring that customer routes are exchanged with the appropriate VPN context.
- ✗
OSPF
Why it's wrong here
OSPF is an interior gateway protocol used for routing within an autonomous system, but it is not used to distribute labels or VPNv4 routes in an MPLS core. While OSPF can be used for underlay routing, it does not carry label information or VPNv4 prefixes. The label distribution and VPN route propagation are handled by LDP and MP-BGP, respectively.
- ✗
BGP-LU
Why it's wrong here
BGP-LU (BGP Labeled Unicast) is an alternative to LDP for distributing labels for IPv4 or IPv6 prefixes. While it can be used in MPLS networks, it is not the standard protocol for label distribution in a typical MPLS L3VPN deployment. The question asks for the protocols used to distribute labels and VPNv4 routes; LDP and MP-BGP are the conventional choices.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
OSPF Neighbor Adjacency and Basics
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.
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.
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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
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.