In IP networking, how does the forwarding of a unicast IP packet differ from the forwarding of a multicast IP packet at a router?

Introduction / Context:
This question compares how routers forward unicast IP packets versus multicast IP packets. Understanding this difference is fundamental for Cisco certification exams because unicast is the default forwarding model in most networks, while multicast uses special forwarding mechanisms and group-based addressing.

Given Data / Assumptions:

• We are working in an IPv4 routed network that supports both unicast and multicast traffic.
• Routers maintain a unicast routing table (RIB and FIB) and, when multicast is enabled, a multicast routing table and group membership information.
• We are focusing on normal, correct operation, not on special tunneling or overlay techniques.

Concept / Approach:
Unicast forwarding is destination based: each packet has a single destination IP address, and the router looks up that address in its routing table to select a single next-hop and outgoing interface. Multicast forwarding is group based: the destination address represents a multicast group, and routers forward packets along a multicast distribution tree only on interfaces that lead to receivers that have joined that group, often using protocols such as IGMP and PIM plus reverse path forwarding checks.

Step-by-Step Solution:
Step 1: For a unicast packet, the router performs a longest-prefix match on the destination IP in the unicast routing table to find one best next-hop and one outgoing interface.Step 2: The router decrements the TTL, rewrites the Layer 2 header, and sends exactly one copy of the packet out that selected interface.Step 3: For a multicast packet, the router identifies the multicast group address (for example 239.x.x.x) and consults its multicast forwarding state to determine which outgoing interfaces are part of the multicast tree.Step 4: The router replicates the packet and sends a copy on each interface in the outgoing interface list, but only toward segments that have interested receivers or downstream routers.Step 5: Reverse path forwarding checks ensure that the multicast traffic is received on the correct incoming interface that matches the unicast route back to the source, helping to prevent loops.

Verification / Alternative check:
In a lab, you can observe unicast routing with commands such as show ip route and multicast forwarding state with show ip mroute. You will see that unicast entries map each prefix to a single next-hop, whereas multicast entries list an incoming interface and one or more outgoing interfaces, confirming the replication behavior.

Why Other Options Are Wrong:
Option B is incorrect because unicast traffic does not flood on all interfaces in a properly configured routed network, and multicast does not send a single copy only toward a default gateway.Option C is wrong because multicast traffic can and often does cross routers when multicast routing is enabled.Option D reverses concepts of source and destination trees and incorrectly claims there is no replication for multicast.

Common Pitfalls:
A common misconception is that multicast behaves like broadcast at Layer 3 and simply floods everywhere. In reality, multicast relies on explicit group membership and tree-building protocols, and routers forward multicast packets only where receivers exist. Another pitfall is thinking that unicast and multicast forwarding use the same routing table; multicast usually relies on additional multicast-specific state and sometimes on reverse path information derived from unicast routes.

Final Answer:
The key distinction is that Unicast forwarding sends one copy of the packet toward a single next-hop based on the destination IP, whereas multicast forwarding may replicate the packet out multiple interfaces along a multicast distribution tree toward all interested receivers in the group.