In computer networks, what is multicast routing?

Introduction / Context:
Many network applications involve one to many or many to many communication, such as live video streaming, online conferencing, stock tickers and distributed simulations. Multicast routing is a technique that allows efficient delivery of the same packet to multiple receivers without sending separate unicast copies to each one. Understanding multicast routing is important for studying advanced IP routing and group communication mechanisms.

Given Data / Assumptions:

We are working with IP multicast, where receivers join specific multicast groups identified by multicast addresses.
Multiple receivers may be scattered across different networks.
The goal is to deliver packets from senders to all group members efficiently.
Routers must cooperate to forward packets along a distribution structure rather than separate unicast paths.

Concept / Approach:
Multicast routing builds a logical distribution tree (or trees) in the network for each multicast group. Receivers signal their interest in a group by joining it, often using protocols like IGMP or MLD on the local link. Multicast routing protocols, such as DVMRP, MOSPF or PIM, use this information along with the unicast routing table to construct paths so that each multicast packet is replicated only where paths diverge toward different receivers. This reduces redundant traffic compared to sending separate unicast copies from the source to each receiver individually.

Step-by-Step Solution:
Step 1: Identify that multicast communication involves one sender to many receivers (one to many) or multiple senders to many receivers (many to many) belonging to a multicast group. Step 2: When a multicast packet is injected into the network, routers must determine which outgoing interfaces lead to group members and forward copies only along those paths. Step 3: Multicast routing protocols coordinate routers to maintain forwarding state for each group, forming a distribution tree connecting all receivers. Step 4: The tree can be source based (one tree per source) or shared (one tree per group), depending on the protocol. Step 5: This clearly distinguishes multicast routing from unicast routing and from simple default gateway forwarding, matching the description in option A.

Verification / Alternative check:
Networking textbooks describe multicast routing as the process of constructing and maintaining multicast distribution trees. They explain algorithms such as Reverse Path Forwarding, pruning and grafting, and PIM modes (Dense and Sparse) that optimize these trees. These descriptions focus on delivering packets to all group members efficiently, not on finding multiple disjoint unicast paths or restricting traffic to a default gateway, confirming the accuracy of option A.

Why Other Options Are Wrong:
Finding multiple disjoint unicast paths for redundancy is related to multipath routing or traffic engineering, not multicast routing.
Traditional client server unicast routing deals with one sender and one receiver per flow and does not use multicast group semantics.
Forwarding only to a default gateway is a simple local routing behavior and does not address group communication across the network.

Common Pitfalls:
Students sometimes think multicast is just repeated unicast, but that would waste bandwidth and processing at intermediate routers. Another pitfall is to confuse broadcast (to all nodes in a broadcast domain) with multicast (to members of a specific group across multiple networks). Multicast routing is more selective and relies on explicit group membership and specialized routing protocols.

Final Answer:
Multicast routing is the process of building and using distribution trees in the network so that packets from one or more sources are efficiently delivered to all receivers that are members of a multicast group.