mediumMultiple Choice
350-401 Practice Question: Runs the following command on Router R1: R1# show…
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
Based on this output, what can be concluded?
⚠ Common exam trap
Cisco often tests the meaning of the 'B' flag in the 'show ip pim neighbor' output, and candidates mistakenly associate it with 'Bootstrap Router' or 'Backup' instead of 'Bidir'.
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
✓
PIM Bidir mode is configured on this interface.
The output shows PIM neighbor modes of 'DR' and 'B'. In PIM Bidir mode, the Designated Router (DR) is elected to forward traffic upstream, and the 'B' flag indicates a Bidir-capable neighbor. This mode is explicitly identified by the 'B' flag in the PIM neighbor table, which is not present in sparse or dense mode. Therefore, the interface is operating in PIM Bidir mode.
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
The B flag in the Mode column explicitly identifies Bidir-PIM, not sparse mode. While Bidir shares the RP and RPF mechanics of sparse mode and also uses a DR on LAN segments, sparse mode itself would never display a B flag; it would show an S flag instead. Thus, the presence of B means the interface is operating under Bidir, making the sparse-mode interpretation incorrect.
- ✓
PIM Bidir mode is configured on this interface.
Why this is correct
The Mode field displays 'B', which is the Cisco IOS indicator for Bidir-PIM. Bidir mode requires a DR election on multi-access networks to forward multicast traffic toward the RP, and here the DR flag is present, confirming that the interface is participating in that election. Since the B flag is specifically reserved for Bidir operation and is not used by any other PIM mode, this is the correct match.
- ✗
PIM dense mode is in use on this interface.
Why it's wrong here
PIM dense mode floods traffic out all interfaces and prunes branches where no receivers exist, so it never performs a DR election, and its interface state would not show a B flag. Dense mode relies on source-based trees and does not use an RP, whereas the B flag and DR presence in the output are consistent with a shared-tree, RP-based design. Therefore, dense mode cannot be in use on this interface.
- ✗
PIM SSM is enabled on this interface.
Why it's wrong here
PIM SSM (Source-Specific Multicast) operates only in the 232.0.0.0/8 range and uses IGMPv3 to signal group membership; it does not use a DR for multicast routing because it skips the shared-tree phase entirely. The interface output shows a DR flag, which is a sign of a shared-tree protocol like Bidir or sparse mode, not SSM. Additionally, SSM would not present a B flag, so this option is inconsistent with the displayed PIM state.
About these practice questions
This 350-401 question is part of Courseiva's 1,923-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.