The answer is that the redistributed BGP routes have a higher administrative distance than the OSPF routes, preventing them from being installed in the routing table and breaking LDP label binding. LDP requires a valid IGP route in the routing table to assign labels for its FECs; when BGP routes are redistributed into OSPF, they become OSPF external routes with a default administrative distance of 110, while the original BGP routes remain at 20 (or 200 for iBGP). Because the router prefers the lower AD, the redistributed routes are not installed, so LDP sees no reachable next-hop for those prefixes and fails to establish neighbor relationships. This scenario tests your understanding of how administrative distance interacts with MPLS label distribution in the Cisco CCNP ENARSI 300-410 exam, often appearing as a tricky redistribution loop where the IGP and BGP feed each other. A common trap is assuming LDP will work as long as routes are in the OSPF database, but LDP only uses the routing table. Memory tip: “LDP needs a table seat, not just a database invite”—if the route isn’t installed, no label is assigned.
300-410 Route Redistribution Practice Question
This 300-410 practice question tests your understanding of route redistribution. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An MPLS network uses OSPF as the IGP. After redistributing BGP routes into OSPF, some LDP neighbors fail to establish. Router R1 config:
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 redistribution of BGP into OSPF may cause OSPF routes to have a high metric or be external, which LDP may not use for label binding if the route is not in the routing table as an IGP route. LDP requires the route to be in the routing table with a next-hop that is reachable via an IGP. If the redistributed routes have a higher administrative distance or are not installed, LDP may fail to assign labels. The fix is to ensure that the IGP routes are preferred or use route-target filtering.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Route Redistribution — This question tests Route Redistribution — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The redistributed BGP routes have a higher administrative distance, causing them to not be installed in the routing table, breaking LDP label binding. — The redistribution of BGP into OSPF may cause OSPF routes to have a high metric or be external, which LDP may not use for label binding if the route is not in the routing table as an IGP route. LDP requires the route to be in the routing table with a next-hop that is reachable via an IGP. If the redistributed routes have a higher administrative distance or are not installed, LDP may fail to assign labels. The fix is to ensure that the IGP routes are preferred or use route-target filtering.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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