hardMultiple Choice
350-401 Practice Question: A service provider is migrating from a…
A service provider is migrating from a traditional IP core to an MPLS core. The engineer has configured LDP on all core routers and verified that LDP sessions are established. However, some prefixes learned via OSPF are not being assigned labels. The 'show mpls ldp bindings' command shows missing bindings for certain routes. What is the most likely cause?
⚠ Common exam trap
Candidates often assume LDP requires a special configuration to work with OSPF, but in reality LDP automatically labels all routes in the global routing table, so missing bindings point to the routes not being present in the RIB rather than a protocol interaction issue.
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 routes are not present in the global routing table on the router.
LDP assigns labels to every prefix present in the global routing table by default. If a prefix is not in the routing table (e.g., because it is a directly connected subnet that is not advertised via OSPF or is filtered by a route-map), LDP will not generate a label binding for it. The 'show mpls ldp bindings' command only displays bindings for routes that exist in the routing table, so missing bindings indicate the routes are absent from the global RIB.
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 routes are not present in the global routing table on the router.
Why this is correct
The correct reason is that LDP binds labels only to prefixes that are actively installed in the global routing table (RIB). In this scenario, even though OSPF may be advertising the routes through neighboring routers, those specific destinations are absent from the router's RIB, so there is no forwarding equivalence class (FEC) for LDP to assign a label binding to. Since LDP is a pure RIB consumer, any route missing from the global table—due to filtering, administrative distance issues, or passive interfaces—will never receive a label.
- ✗
The OSPF process is not redistributed into LDP.
Why it's wrong here
This is incorrect because LDP does not use a redistribution mechanism analogous to routing protocol redistribution. LDP directly reads the global routing table to construct its FECs, and it does not care whether a route was learned via OSPF, EIGRP, or BGP. Redistribution applies only when moving routes between different routing protocols; LDP has no protocol input other than the RIB itself, so 'not redistributing OSPF into LDP' is architecturally meaningless and cannot explain missing labels.
- ✗
LDP is configured to only assign labels to BGP routes.
Why it's wrong here
This is incorrect because LDP's default behavior is to assign labels to every route in the global routing table, including IGP routes such as OSPF prefixes. To restrict LDP to only BGP routes, an explicit label-filtering policy (e.g., using an access-list matching only BGP routes) would need to be configured under the LDP process or interface. Without such a policy, LDP cannot be limited to BGP alone; the presence of OSPF routes in the RIB would still result in label bindings for them, so this is not a plausible cause.
- ✗
The 'mpls ldp autoconfig' command is missing on OSPF.
Why it's wrong here
This is incorrect because the `mpls ldp autoconfig` command, when enabled under the OSPF process, simply automates the enabling of LDP on all OSPF-enabled interfaces. It does not control which routes get label bindings; label assignment is determined solely by the presence of routes in the global RIB. Even if autoconfig is missing, manually enabling `mpls ip` on the relevant interfaces would still allow LDP to label routes present in the RIB, and conversely, autoconfig cannot label routes that are absent from the routing table.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 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 350-401 exam.