Which of the following statements about multicast packet forwarding is true?

Question

Options

• AIf a multicast data packet fails the RPF check, the packet must have been received through a sub-optimal interface. However, this interface still receives and forwards the packet.
• BIGMP snooping cannot control the scope of multicast traffic flooding on a Layer 2 network.
• CThe source address of a multicast packet is a unicast address.
• DIn multicast transmission, the destination address of a packet can be the unicast address of a host.

• A
  5% (1)
• B
  5% (1)
• C
  81% (17)
• D
  10% (2)

Explanation

Multicast communication always requires a real sender, and every real sender is an ordinary host with a unicast IP address. Only the destination field in the IP header uses a Class D multicast group address (224.0.0.0-239.255.255.255), which identifies the group of receivers; the source field always holds the unicast address of the originating host, making C correct.

Option A is wrong on two counts: a failed RPF check does not mean the interface is sub-optimal -- it means the packet arrived on the wrong interface relative to the best return path to the source -- and crucially, a packet that fails the RPF check is dropped, never forwarded. Option B is wrong because IGMP snooping exists precisely to limit multicast flooding on Layer 2; by inspecting IGMP join/leave messages, a switch learns which ports need each multicast stream and forwards only to those ports instead of flooding every port. Option D is wrong because using a unicast destination address would make the transmission unicast by definition; multicast transmission always uses a multicast group address as the destination.

Memory tip: Source equals sender, and every sender is one specific machine with one specific unicast address. Only the destination gets the group address because it represents many receivers at once.

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