The root cause is that the OSPF summary route suppresses the more specific /24 prefixes from the routing table, so LDP does not assign labels to them, causing the MPLS LDP label distribution failure. When R1 uses the `area 0 range 10.0.0.0 255.255.252.0` command, OSPF installs only the aggregate 10.0.0.0/22 route, and LDP binds a label only to that summary entry. R2 then forwards traffic for 10.0.1.0/24 toward R1 with a label for the /22, but R1 lacks a label for the specific /24, resulting in a label mismatch or blackholing. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how OSPF summarization interacts with LDP label assignment—a common trap is assuming LDP automatically labels all reachable subnets, when in fact it only labels routes present in the FIB. Remember the memory tip: "If the route is not in the table, LDP is not able."
300-410 Route Summarization Practice Question
This 300-410 practice question tests your understanding of route summarization. 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.
MPLS LDP neighbor mismatch is causing label distribution failures for summary routes. Router R1 and R2 are LDP peers. R1 has:
mpls ip
!
interface GigabitEthernet0/0
ip address 10.0.0.1 255.255.255.0
mpls ip
!
router ospf 1
network 10.0.0.0 0.0.0.255 area 0
area 0 range 10.0.0.0 255.255.252.0
Local tag outgoing tag prefix tag(s) next-hop
16 Untagged 10.0.0.0/22 0 10.0.0.1
But R2 cannot forward traffic for 10.0.1.0/24. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The OSPF summary suppresses the /24 routes, so LDP does not assign labels to them, causing forwarding failures.
The OSPF area range command creates a summary route 10.0.0.0/22 in the routing table, but LDP assigns labels only to routes that are in the routing table. The more specific /24 routes are suppressed by the summary, so LDP does not have labels for them. When R2 receives a packet for 10.0.1.0/24, it looks up the route, finds the summary, and forwards it to R1 with a label, but R1 may not have a label for the specific /24, causing a label mismatch or blackholing. The fix is to not summarize or to use label binding for more specific prefixes.
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 OSPF summary suppresses the /24 routes, so LDP does not assign labels to them, causing forwarding failures.
Why this is correct
LDP only labels routes present in the routing table; the /24 routes are missing.
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.
Trap categories for this question
Command / output trap
No static route is shown.
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 Summarization — This question tests Route Summarization — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The OSPF summary suppresses the /24 routes, so LDP does not assign labels to them, causing forwarding failures. — The OSPF area range command creates a summary route 10.0.0.0/22 in the routing table, but LDP assigns labels only to routes that are in the routing table. The more specific /24 routes are suppressed by the summary, so LDP does not have labels for them. When R2 receives a packet for 10.0.1.0/24, it looks up the route, finds the summary, and forwards it to R1 with a label, but R1 may not have a label for the specific /24, causing a label mismatch or blackholing. The fix is to not summarize or to use label binding for more specific prefixes.
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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