hardMultiple Choice
300-410 An MPLS network uses OSPF as the IGP Practice Question
An MPLS network uses OSPF as the IGP. After redistributing BGP routes into OSPF, some MPLS forwarding failures occur for the redistributed prefixes. Router R1 config:
router ospf 1
redistribute bgp 65001 subnets !
router bgp 65001
redistribute ospf 1
R1# show mpls ldp neighbor
Peer LDP Ident: 10.1.1.2:0, Local LDP Ident: 10.1.1.1:0 TCP connection: 10.1.1.2.646 - 10.1.1.1.646 State: Oper, Msg sent: 100, Msg rcvd: 80 Downstream on demand
R2# show mpls ldp neighbor
Peer LDP Ident: 10.1.1.1:0, Local LDP Ident: 10.1.1.2:0 TCP connection: 10.1.1.1.646 - 10.1.1.2.646 State: Oper, Msg sent: 80, Msg rcvd: 100
What is the root cause?
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
✓
The redistributed BGP routes have a higher administrative distance, causing them to not be installed in the routing table, breaking LDP label binding.
The show commands display an operational LDP session; however, the issue is that redistributed BGP routes become OSPF external routes with administrative distance 110. If the same prefix is also learned via eBGP (AD 20), the eBGP route is preferred and installed in the routing table. LDP requires the route to be in the routing table to assign a label. Since the OSPF external route is not installed, LDP does not assign a label for that prefix, which can cause MPLS forwarding failures for those prefixes. The LDP neighbor relationships themselves remain up, but label binding for the redistributed prefixes is broken. To fix, either adjust administrative distances (e.g., use `distance bgp 120 120 120` to make BGP less preferred) or use route filtering to avoid creating duplicate routes.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The redistributed BGP routes have a higher administrative distance, causing them to not be installed in the routing table, breaking LDP label binding.
Why this is correct
LDP uses the routing table; if the route is not installed, LDP cannot assign a label.
- ✗
The LDP router-id is misconfigured, causing neighbor failure.
Why it's wrong here
LDP neighbors are operational; the issue is label binding.
- ✗
The OSPF process is missing the mpls ldp autoconfig command.
Why it's wrong here
LDP autoconfig is not required; LDP can work with OSPF without it.
- ✗
The BGP redistribution is missing the route-map to set the metric.
Why it's wrong here
Missing route-map does not prevent LDP neighbor formation.
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
MPLS Label Distribution
MPLS Label Distribution is the process by which routers exchange labels that tell them how to forward packets across a network without looking at the IP address each time.
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 by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.