350-501 Networking Practice Question
During multicast troubleshooting, the command 'show ip mroute' shows that the RPF neighbor for a particular (S,G) entry is incorrect. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that multicast issues are caused by multicast-specific misconfigurations (like PIM or IGMP), when in fact the root cause is a unicast routing problem, specifically the absence of a route back to the source in the unicast routing table.
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 unicast routing table does not have a route back to the source.
The RPF (Reverse Path Forwarding) neighbor for an (S,G) entry is determined by the unicast routing table: the router expects to reach the source S via the interface and next-hop that the unicast route provides. If the unicast routing table does not have a route back to the source, the router cannot determine the correct RPF interface and neighbor, leading to an incorrect RPF neighbor in the 'show ip mroute' output. This is the most common cause of RPF failures in multicast troubleshooting.
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 unicast routing table does not have a route back to the source.
Why this is correct
RPF check uses unicast route towards source; missing or incorrect route leads to wrong RPF neighbor.
- ✗
The multicast routing protocol is not enabled globally.
Why it's wrong here
If not enabled, no multicast entries would exist.
- ✗
TTL threshold is set too high on the incoming interface.
Why it's wrong here
TTL threshold would drop packets, not affect RPF neighbor.
- ✗
IGMP snooping is disabled on the switch.
Why it's wrong here
IGMP snooping is layer 2; does not affect RPF on routers.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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