Questions: Multicast Routing Protocols

Question 1

500 hosts are receiving a multicast video stream. A link in the distribution tree connects a router to two downstream routers, each of which has receivers. How many copies of each packet travel over that link?

• A. 500 copies — one per receiver reachable through that link
• B. Two copies — one per downstream router
• C. One copy — the downstream router receiving it will duplicate the packet toward its own branches
• D. Zero copies — multicast packets are never forwarded over internal links

Answer: (C)

This is the core efficiency of multicast: each link carries at most one copy of each packet. The router at the branching point receives one copy and replicates it only there — sending one copy down each outgoing branch. Traffic on the link equals one copy regardless of how many receivers sit downstream. If each receiver required its own stream, this single link would carry 500 copies (or however many receivers are downstream), wasting orders of magnitude more bandwidth.

Question 2

A large enterprise WAN has thousands of routers but only 50 hosts subscribed to a particular multicast group. Which PIM mode is appropriate, and why?

• A. PIM Dense Mode, because flooding reaches all 50 receivers immediately without requiring them to join explicitly
• B. PIM Sparse Mode, because receivers are spread thinly and explicit join messages ensure traffic flows only to paths where receivers actually exist
• C. PIM Dense Mode, because shared trees use less state than source-specific trees
• D. Neither mode; multicast cannot function across a WAN

Answer: (B)

PIM-SM is designed for exactly this scenario. 'Sparse' refers to the density of receivers relative to the total network — 50 receivers among thousands of routers is very sparse. Explicit join messages propagate only toward receivers who request traffic, so the vast majority of routers never see the multicast traffic. PIM-DM would flood the entire network first and then prune, which wastes bandwidth proportional to network size. Option C mistakes the tree types — sparse mode can use both shared trees and source-specific trees.

Question 3

In PIM Sparse Mode, a multicast session begins with traffic flowing via the rendezvous point, but can later switch to a source-specific tree once active traffic is detected.

• T. True
• F. False

Answer: True

PIM-SM's two-phase design is intentional. The rendezvous point provides a common meeting place for sources and receivers with low setup cost, before anyone knows whether the stream will even be used. Once traffic flows and the receiver's router detects it, it can send a Join directly toward the source, cutting out the rendezvous point for a shorter, more efficient path. This balances setup simplicity against long-term efficiency.

Question 4

PIM (Protocol Independent Multicast) runs its own separate routing algorithm to compute the network topology for building multicast distribution trees.

• T. True
• F. False

Answer: False

The 'Protocol Independent' in PIM is a direct statement that it does NOT run its own routing algorithm. Instead, PIM consults whatever unicast routing table is already populated by OSPF, BGP, or another protocol, and uses those routes to determine the reverse-path toward the source. This is why PIM can be deployed on top of any unicast routing infrastructure without replacing it — a key design advantage.

Question 5

Explain how multicast routing reduces bandwidth compared to individual unicast streams, and where packet duplication actually occurs.

Model answer: With unicast, the source sends N separate copies — one per receiver — so any link on the path to all receivers carries N copies. With multicast, the source sends one copy into the network, and routers replicate it only at branching points in the distribution tree where the tree forks toward different receivers. Each link carries at most one copy. Duplication happens only where necessary, at the router where two or more downstream branches diverge. The result is O(1) packets per link rather than O(N) at the source.

The bandwidth reduction is most dramatic when many receivers share a common upstream path. In the lecture-streaming example, a single campus backbone link carries one copy regardless of whether 5 or 5,000 students are watching. Without multicast, that link would carry 5,000 copies. The tree structure ensures minimum replication: each link and each router does only the work that is strictly necessary to reach its downstream receivers.