What is one of the purposes of a default Multicast Distribution Tree?

Question

Options

• Ato handle high-bandwidth source streams
• Bto define the RP for PIM sparse mode and the different leafs
• Cto offload traffic from data MDT
• Dto support control traffic between different VRFs

• A
  5% (3)
• B
  7% (4)
• C
  73% (44)
• D
  15% (9)

Explanation

Multicast Distribution Trees (MDT) - Exam Breakdown

Option C is correct because in MPLS MVPN, the Default MDT is a permanent, shared tree that all PE routers in a VPN join to carry control plane traffic and low-bandwidth multicast streams. Since it handles this baseline traffic, high-bandwidth streams can be offloaded to a Data MDT only when needed - meaning the Default MDT absorbs the "routine" load so Data MDTs don't have to be created for every stream.

Why the distractors are wrong:

• A - High-bandwidth streams are handled by the Data MDT, not the Default MDT. When a source exceeds a configured bandwidth threshold, traffic is specifically moved away from the Default MDT to a Data MDT.
• B - Defining the Rendezvous Point (RP) and PIM sparse mode topology is a function of PIM configuration, not a purpose of the Default MDT itself.
• D - The Default MDT carries control traffic within the same VPN across PE routers. "Between different VRFs" mischaracterizes its scope and makes this factually incorrect.

Memory tip: Think Default = always-on bus, Data = express lane. The Default MDT is always running in the background; a Data MDT only spins up when traffic gets heavy enough to need its own dedicated tree. If you see "high-bandwidth," think Data MDT - not Default.

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