hardMultiple ChoiceObjective-mapped
300-410 Practice Question: MPLS LDP neighbor mismatch is causing label…
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
! R2 shows:
R2# show mpls ldp neighbor
Peer LDP Ident: 10.0.0.2:0, Transport address: 10.0.0.2 TCP connection: 10.0.0.2.646 - 10.0.0.1.646 State: Oper, Msgs sent/rcvd: 10/10
R2# show mpls forwarding-table 10.0.0.0/22
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 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 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.
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.
- ✗
LDP is not enabled on the interface, so no labels are exchanged.
Why it's wrong here
LDP is enabled and neighbors are operational.
- ✗
The OSPF area range command is not supported with MPLS.
Why it's wrong here
It is supported but can cause issues.
- ✗
R2 has a static route for 10.0.1.0/24 that overrides LDP.
Why it's wrong here
No static route is shown.
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
Introduction to ENARSI Exam and Network Fundamentals
Key term
OSPF Route Summarization
OSPF route summarization is a technique that combines multiple smaller network routes into a single, larger route advertisement to reduce the size of routing tables and improve network stability.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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