300-320 Question #227: Real Exam Question with Answer & Explanation

The correct answer is A: multicast routing uses RPF.. Multicast routing uses Reverse Path Forwarding (RPF) to prevent routing loops - the router checks that incoming multicast packets arrive on the interface used to reach the source, and drops them if not. This also means the source address of the packet must be known (choice C), wh

Question

Which two of these are characteristics of multicast routing? (Choose two.)

Options

Amulticast routing uses RPF.
Bmulticast routing is connectionless.
CIn multicast routing, the source of a packet is known.
DWhen network topologies change, multicast distribution trees are not rebuilt, but use the original
EMulticast routing is much like unicast routing, with the only difference being that it has a a group

Explanation

Multicast routing uses Reverse Path Forwarding (RPF) to prevent routing loops - the router checks that incoming multicast packets arrive on the interface used to reach the source, and drops them if not. This also means the source address of the packet must be known (choice C), which is a fundamental requirement for RPF to function. Choice B is incorrect: multicast routing is not connectionless in the way the option implies - RPF adds source-awareness. Choice D is incorrect: multicast distribution trees ARE rebuilt when the network topology changes. Choice E is incorrect: multicast routing differs from unicast in many important ways beyond just having a group address - it uses special protocols (PIM, IGMP), shared/shortest-path trees, and RPF-based forwarding.

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