Question 1,896 of 2,015

MPLS →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the LDP session is operational, using TCP port 646 for LDP and port 179 for BGP, with the session in the Oper state. This is correct because the show mpls ldp neighbor output interpretation clearly shows the State as "Oper" (operational), and the TCP connection line lists port 646 for the peer and port 179 for the local router—port 646 is the well-known port for Label Distribution Protocol, while port 179 is used for BGP, which often runs alongside LDP on the same routers. On the ENCOR 350-401 exam, this output tests your ability to quickly verify MPLS LDP neighbor adjacency status and distinguish LDP from BGP ports, a common trap being that students confuse port 179 as an LDP port. A key memory tip: think "646 for Labels, 179 for Paths"—LDP uses 646 to distribute labels, while BGP uses 179 to exchange routing paths.

350-401 MPLS Practice Question

This 350-401 practice question tests your understanding of mpls. 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.

A network engineer runs the following command on Router R1:

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 - 10.1.1.1.179 State: Oper; Msgs sent/rcvd: 120/115; Downstream Up time: 02:30:15 LDP discovery sources: GigabitEthernet0/0, Src IP addr: 192.168.1.2 Addresses bound to peer LDP Ident:

10.1.1.2         192.168.1.2      10.2.2.2

Based on this output, what can be concluded?

Question 1mediummultiple choice

Read the full MPLS explanation →

A
The LDP session is using TCP port 646 for LDP and port 179 for BGP, and the session is operational.
State: Oper confirms the session is up, and TCP ports 646 (LDP) and 179 (BGP) are correctly identified.
B
The LDP session is down because the state is 'Oper' but the TCP connection shows port 179.
Why wrong: State: Oper means operational; port 179 is the BGP port, but here it's shown as the local port for LDP (which is incorrect in real life, but the question tests reading the output as given).
C
The LDP neighbor is using label distribution mode 'Upstream' based on the output.
Why wrong: The output explicitly says 'Downstream', not 'Upstream'.
D
The LDP session has been up for 2 hours and 30 minutes, and the peer has sent 115 messages.
Why wrong: The uptime is correct, but the peer has sent 115 messages (Msgs sent/rcvd: 120/115 means local sent 120, received 115).

Full breakdown with real-world context →

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 LDP session is using TCP port 646 for LDP and port 179 for BGP, and the session is operational.

The output shows an established LDP session (State: Oper) between LSRs 10.1.1.1 and 10.1.1.2, with TCP port 646 (LDP) and port 179 (BGP) shown. The 'Downstream' label distribution mode is indicated. The peer's addresses include loopback and interface IPs.

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 LDP session is using TCP port 646 for LDP and port 179 for BGP, and the session is operational.
Why this is correct
State: Oper confirms the session is up, and TCP ports 646 (LDP) and 179 (BGP) are correctly identified.
Related concept
OSPF neighbours must agree on key parameters.
✗
The LDP session is down because the state is 'Oper' but the TCP connection shows port 179.
Why it's wrong here
State: Oper means operational; port 179 is the BGP port, but here it's shown as the local port for LDP (which is incorrect in real life, but the question tests reading the output as given).
✗
The LDP neighbor is using label distribution mode 'Upstream' based on the output.
Why it's wrong here
The output explicitly says 'Downstream', not 'Upstream'.
✗
The LDP session has been up for 2 hours and 30 minutes, and the peer has sent 115 messages.
Why it's wrong here
The uptime is correct, but the peer has sent 115 messages (Msgs sent/rcvd: 120/115 means local sent 120, received 115).

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.

Trap categories for this question

Command / output trap
State: Oper means operational; port 179 is the BGP port, but here it's shown as the local port for LDP (which is incorrect in real life, but the question tests reading the output as given).

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.

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 350-401 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 350-401 question test?

MPLS — This question tests MPLS — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The LDP session is using TCP port 646 for LDP and port 179 for BGP, and the session is operational. — The output shows an established LDP session (State: Oper) between LSRs 10.1.1.1 and 10.1.1.2, with TCP port 646 (LDP) and port 179 (BGP) shown. The 'Downstream' label distribution mode is indicated. The peer's addresses include loopback and interface IPs.

What should I do if I get this 350-401 question wrong?

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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