hardMultiple Choice
350-401 Practice Question: Issues the following command on Router R7: R7#…
A network engineer issues the following command on Router R7:
R7# show ip pim tunnel
Tunnel1: Type: PIM Encap Source: 10.0.0.7, Destination: 10.0.0.8 Status: up
Based on this output, what can be concluded?
⚠ Common exam trap
Cisco often tests the distinction between PIM register encapsulation (used for source-to-RP communication) and other tunnel types like MDT or Auto-RP, leading candidates to confuse the purpose of the PIM Encap tunnel.
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
✓
This tunnel is used for PIM register encapsulation to the RP.
The 'show ip pim tunnel' output displays a PIM Encap tunnel, which is automatically created by Cisco IOS to encapsulate multicast packets from a source that is outside the Rendezvous Point (RP) tree. The tunnel sends these encapsulated packets to the RP (10.0.0.8) using unicast PIM register messages, making option A correct.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
This tunnel is used for PIM register encapsulation to the RP.
Why this is correct
In Protocol Independent Multicast Sparse Mode (PIM-SM), when a source sends multicast traffic to a group with unknown or no downstream receivers, the first-hop router (FHR) encapsulates each original multicast packet into a unicast PIM Register message and sends it to the rendezvous point (RP). This tunnel interface, often named Tunnel0, serves as the virtual point-to-point link for that encapsulation, carrying the multicast payload inside unicast PIM packets. The RP decapsulates these messages, learns the source's existence, and then can join the shortest path tree toward the source.
- ✗
This tunnel is used for MDT data group forwarding.
Why it's wrong here
Multicast Distribution Tree (MDT) data group forwarding applies exclusively to MVPN (Multicast VPN) solutions, where service providers use a default MDT or data MDT to carry customer multicast traffic across the MPLS backbone. In contrast, the tunnel shown here is created by the PIM-SM protocol specifically to deliver Register messages to the RP, not to forward multicast data groups. Mixing these two concepts is a common mistake because both involve 'group' addressing, but MDT tunnels operate on provider-edge routers in an MVPN context, not on a normal PIM network.
- ✗
This tunnel is used for BSR messages.
Why it's wrong here
Bootstrap Router (BSR) messages are part of the PIM-SM RP discovery mechanism, sent periodically by the elected BSR as link-local multicast packets (destination 224.0.0.19) and flooded hop-by-hop through the network. They are never encapsulated or tunneled, because the BSR protocol relies on each PIM neighbor receiving BSM on the multicast address directly. The tunnel interface in question is used solely for the encapsulation of data packets in Register messages, not for the BSR control-plane traffic that announces RP-to-group mappings.
- ✗
This tunnel is used for Auto-RP announcements.
Why it's wrong here
Auto-RP announcements are a legacy Cisco RP discovery mechanism that uses multicast groups 224.0.1.39 (Announce) and 224.0.1.40 (Discover) to distribute RP information. These messages are forwarded natively via multicast, not over a tunnel, since they are part of the PIM control plane and must reach all routers in the domain dynamically. The purpose of the tunnel referenced in the exhibit is the transport of multicast data packets via PIM Register encapsulation to the RP, which is an entirely different function from Auto-RP's control-plane group updates.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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