---
title: "Multicast and unicast routing protocols"
subject: "Computer Networks and Security"
chapter: 4
key: "semester_7/computer_networks_and_security:multicast-unicast-routing"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/multicast-unicast-routing
site: BCT Engineering
---

# Multicast and unicast routing protocols

Computer Networks and Security · Chapter 4: Network Layer

- Asked in: 2072 Kartik, 6 marks
- Video: [Lec-61: What is Routing Protocols | Various types of Routing Protocols](https://www.youtube.com/watch?v=rA0p0ouD3aE) — Gate Smashers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/multicast-unicast-routing
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-4

## Question

Explain briefly about multicast routing protocols and unicast routing protocols.

## Short answer

Unicast routing (RIP, OSPF, BGP) delivers one-to-one using distance-vector, link-state, or path-vector algorithms. Multicast routing delivers one-to-many to group addresses (224.0.0.0/4) using IGMP for membership and protocols like PIM-SM/DM, DVMRP, MOSPF, CBT to build shared or source-based trees, saving bandwidth by replicating only at branch points.

## Answer

Routing protocols decide how a router builds its table. **Unicast** routing serves one source to one destination; **multicast** routing serves one source to a group of receivers.

#### Unicast routing protocols
Used for one-to-one delivery. They are grouped by the way routes are computed.

| Protocol | Type | Key points |
|---|---|---|
| RIP | Distance vector | Bellman-Ford; metric is hop count (max 15); updates every 30 s; slow convergence |
| OSPF | Link state | Each router floods link-state advertisements, runs Dijkstra; cost metric; areas; fast convergence |
| BGP | Path vector | Between autonomous systems (inter-domain); policy based; runs over TCP port 179 |

- **Intra-domain** (inside an AS): RIP, OSPF. **Inter-domain:** BGP.

#### Multicast routing protocols
Packets are sent once and copied by routers only where the paths to group members split. Group addresses are in 224.0.0.0/4 (Class D). **IGMP** lets a host tell its local router which group it joins; the routing protocols then build a delivery tree.

| Protocol | Idea |
|---|---|
| DVMRP | Distance vector; flood and prune with reverse path forwarding; source-based trees |
| MOSPF | Extension of OSPF; every router knows the group membership; source-based trees |
| PIM-DM | Flood and prune, uses the existing unicast table |
| PIM-SM | Group-shared tree rooted at a rendezvous point (RP); suits sparse groups |
| CBT | Core-based tree shared by all senders |

```text
Source -> R1 -> R2 -> Host in group
            \-> R3 -> Host in group   (one copy per link)
```

Multicast saves bandwidth compared with sending many unicast copies.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does multicast save bandwidth?
   A. Sending a copy to each member
   B. One copy sent, duplicated only where paths split
   C. Broadcasting
   D. Compression
   Answer: B. Routers copy only at branches. (Key point: Multicast idea)

2. What is IGMP's role?
   A. Routes between ASs
   B. Lets a host tell its local router which group it joins
   C. Assigns IPs
   D. Builds OSPF
   Answer: B. IGMP handles group membership. (Key point: IGMP)

3. PIM-SM uses:
   A. Flood and prune
   B. A shared tree rooted at a rendezvous point
   C. Dijkstra only
   D. Hop counts
   Answer: B. Suits sparse groups. (Key point: PIM-SM)

4. Multicast group addresses are in:
   A. 10.0.0.0/8
   B. 224.0.0.0/4
   C. 192.168.0.0/16
   D. 240.0.0.0/4
   Answer: B. Class D. (Key point: Group addresses)

## Related in other subjects

- [Global unicast, link local, site local, multicast](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aipv6-address-types) — Internet and Intranet, chapter 2