Question 960 of 2,152

Route Redistribution →hardMultiple ChoiceObjective-mapped

LDP Neighbor Failure After BGP-OSPF Redistribution — Administrative Distance Issue | Cisco CCNP ENARSI 300-410 Explained

This 300-410 practice question tests your understanding of route redistribution. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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:

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?

A
The redistributed BGP routes have a higher administrative distance, causing them to not be installed in the routing table, breaking LDP label binding.
LDP uses the routing table; if the route is not installed, LDP cannot assign a label.
B
The LDP router-id is misconfigured, causing neighbor failure.
Why wrong: LDP neighbors are operational; the issue is label binding.
C
The OSPF process is missing the mpls ldp autoconfig command.
Why wrong: LDP autoconfig is not required; LDP can work with OSPF without it.
D
The BGP redistribution is missing the route-map to set the metric.
Why wrong: Missing route-map does not prevent LDP neighbor formation.

Full breakdown with real-world context →

Quick Answer

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.

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.

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.
Related concept
OSPF neighbours must agree on key parameters.
✗
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.

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.

What to study next

Got this wrong? Here's your next step.

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.

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FAQ

Questions learners often ask

What does this 300-410 question test?

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 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.

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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