The answer is that PIM Bidir mode is configured on this interface. This conclusion is drawn from the `show ip pim neighbor` output, where the Mode column shows one neighbor as DR (Designated Router) and the other as B (Bidir-capable). In standard PIM Sparse Mode, only one DR exists per interface, and the B flag is not present; the simultaneous appearance of both DR and B on the same interface is a hallmark of Bidir mode, which uses a different DR election process and requires the B flag to indicate Bidir capability. On the ENCOR 350-401 exam, this question tests your ability to interpret PIM neighbor table details, often as a trap where candidates mistakenly think the B flag means backup or border router. Remember that in Bidir mode, the DR is elected but all routers must be Bidir-capable, so the B flag confirms the mode. A quick memory tip: “B for Bidir, not backup” — if you see a B in the Mode column, think Bidir mode is active.
350-401 IP Multicast Practice Question
This 350-401 practice question tests your understanding of ip multicast. 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 ip pim neighbor
PIM Neighbor Table
Neighbor Address Interface Uptime Expires Mode
10.1.1.2 GigabitEthernet0/0 2w0d 00:01:25 DR
10.1.1.3 GigabitEthernet0/0 2w0d 00:01:20 B
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
PIM Bidir mode is configured on this interface.
The 'show ip pim neighbor' output shows two neighbors on the same interface. The 'Mode' column indicates the PIM neighbor role: 'DR' means Designated Router, 'B' means Bidir-capable. The presence of both DR and B on the same interface suggests that PIM Bidir mode is enabled, as Bidir uses a different DR election and the B flag indicates Bidir capability. The correct answer is that PIM Bidir is configured.
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.
✗
PIM sparse mode is operating on this interface.
Why it's wrong here
In sparse mode, the DR flag would be present, but the B flag indicates Bidir.
✓
PIM Bidir mode is configured on this interface.
Why this is correct
The B flag in the Mode column indicates Bidir capability, and the DR flag is also present, which is consistent with Bidir operation.
Dense mode does not use DR election or show B flag.
✗
PIM SSM is enabled on this interface.
Why it's wrong here
SSM does not use DR election; it relies on IGMPv3.
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
Dense mode does not use DR election or show B flag.
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 350-401 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 350-401 OSPF questions on adjacency and route selection.
IP Multicast — This question tests IP Multicast — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: PIM Bidir mode is configured on this interface. — The 'show ip pim neighbor' output shows two neighbors on the same interface. The 'Mode' column indicates the PIM neighbor role: 'DR' means Designated Router, 'B' means Bidir-capable. The presence of both DR and B on the same interface suggests that PIM Bidir mode is enabled, as Bidir uses a different DR election and the B flag indicates Bidir capability. The correct answer is that PIM Bidir is configured.
What should I do if I get this 350-401 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 350-401 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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