- A
LDP will use the IP address of GigabitEthernet0/0 as the transport address because Loopback0 is not in OSPF.
Why wrong: Loopback0 is advertised via OSPF because the network command includes 10.0.0.0/0.0.0.3, but the loopback is 192.168.0.1/32, which is not covered. Actually, the loopback is not in OSPF, so the neighbor may not have a route to it, causing LDP session failure. This is a subtle issue.
- B
LDP will use Loopback0 (192.168.0.1) as the transport address, but the neighbor must have a route to 192.168.0.1 for the session to establish.
The 'force' keyword sets the LDP router ID to Loopback0. However, the loopback is not included in the OSPF network statements, so the neighbor may not have reachability to 192.168.0.1, causing LDP session failure. This is a common misconfiguration.
- C
LDP will not use Loopback0 because the interface is not configured with 'mpls ip'.
Why wrong: The LDP router ID does not require 'mpls ip' on the loopback; it is only a transport address.
- D
LDP will use the IP address of GigabitEthernet0/1 because it has the highest IP among MPLS-enabled interfaces.
Why wrong: The 'force' keyword overrides automatic selection; LDP will use Loopback0 regardless.
300-410 LDP Transport Address Practice Question
This 300-410 practice question tests your understanding of mpls operations. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: lDP Transport Address. 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.
Consider the following configuration snippet:
ip cef
!
interface GigabitEthernet0/0 ip address 10.0.0.1 255.255.255.252
mpls ip !
interface GigabitEthernet0/1 ip address 10.0.1.1 255.255.255.252
mpls ip !
router ospf 1 network 10.0.0.0 0.0.0.3 area 0 network 10.0.1.0 0.0.0.3 area 0
! mpls ldp router-id Loopback0 force !
interface Loopback0 ip address 192.168.0.1 255.255.255.255
What will happen when this router attempts to establish an LDP session with a neighbor on GigabitEthernet0/0?
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
LDP will use Loopback0 (192.168.0.1) as the transport address, but the neighbor must have a route to 192.168.0.1 for the session to establish.
The configuration sets the LDP router-id to Loopback0 (192.168.0.1) using the 'mpls ldp router-id Loopback0 force' command. This also becomes the LDP transport address. However, the loopback interface is not included in OSPF (the OSPF network statements only cover the two point-to-point links, 10.0.0.0/30 and 10.0.1.0/30). Therefore, unless the neighbor has an alternate route to 192.168.0.1 (e.g., via another routing protocol or static route), the LDP TCP session cannot be established because the neighbor cannot reach the transport address.
Key principle: LDP Transport Address
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
LDP will use the IP address of GigabitEthernet0/0 as the transport address because Loopback0 is not in OSPF.
Why it's wrong here
Loopback0 is advertised via OSPF because the network command includes 10.0.0.0/0.0.0.3, but the loopback is 192.168.0.1/32, which is not covered. Actually, the loopback is not in OSPF, so the neighbor may not have a route to it, causing LDP session failure. This is a subtle issue.
- ✓
LDP will use Loopback0 (192.168.0.1) as the transport address, but the neighbor must have a route to 192.168.0.1 for the session to establish.
Why this is correct
The 'force' keyword sets the LDP router ID to Loopback0. However, the loopback is not included in the OSPF network statements, so the neighbor may not have reachability to 192.168.0.1, causing LDP session failure. This is a common misconfiguration.
Related concept
LDP Transport Address
- ✗
LDP will not use Loopback0 because the interface is not configured with 'mpls ip'.
Why it's wrong here
The LDP router ID does not require 'mpls ip' on the loopback; it is only a transport address.
- ✗
LDP will use the IP address of GigabitEthernet0/1 because it has the highest IP among MPLS-enabled interfaces.
Why it's wrong here
The 'force' keyword overrides automatic selection; LDP will use Loopback0 regardless.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The loopback interface is not advertised in OSPF, so the neighbor may not have a route to the LDP router ID.
Trap categories for this question
Keyword trap
The 'force' keyword overrides automatic selection; LDP will use Loopback0 regardless.
Command / output trap
Loopback0 is advertised via OSPF because the network command includes 10.0.0.0/0.0.0.3, but the loopback is 192.168.0.1/32, which is not covered. Actually, the loopback is not in OSPF, so the neighbor may not have a route to it, causing LDP session failure. This is a subtle issue.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- LDP Transport Address
- Router ID
- Loopback Interface
- IGP Route Requirement
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
LDP Transport Address
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
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FAQ
Questions learners often ask
What does this 300-410 question test?
MPLS Operations — This question tests MPLS Operations — LDP Transport Address.
What is the correct answer to this question?
The correct answer is: LDP will use Loopback0 (192.168.0.1) as the transport address, but the neighbor must have a route to 192.168.0.1 for the session to establish. — The configuration sets the LDP router-id to Loopback0 (192.168.0.1) using the 'mpls ldp router-id Loopback0 force' command. This also becomes the LDP transport address. However, the loopback interface is not included in OSPF (the OSPF network statements only cover the two point-to-point links, 10.0.0.0/30 and 10.0.1.0/30). Therefore, unless the neighbor has an alternate route to 192.168.0.1 (e.g., via another routing protocol or static route), the LDP TCP session cannot be established because the neighbor cannot reach the transport address.
What should I do if I get this 300-410 question wrong?
Review lDP Transport Address, then practise related 300-410 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
LDP Transport Address
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Last reviewed: Jun 18, 2026
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