3.4 IP Services
📘CCNP Encore (350-401-ENCORE-v1.1)
1. What is Multicast?
Multicast is a way to send a single stream of data to multiple devices at once without sending separate copies to each device.
- Example in IT networks:
- Streaming a software update to 50 servers simultaneously.
- Sending a live network monitoring feed to multiple monitoring tools.
Key difference from other types of communication:
- Unicast: 1-to-1 (like normal web browsing).
- Broadcast: 1-to-all on the same network.
- Multicast: 1-to-many, only to devices that want the data.
2. Reverse Path Forwarding (RPF) Check
RPF is a safety mechanism that ensures multicast traffic only travels the correct path toward the source.
How it works:
- Router receives a multicast packet.
- Router checks: “Is the interface this packet came from the best path back to the source?”
- This is based on the router’s unicast routing table.
- If yes → packet is forwarded to other interfaces.
If no → packet is dropped.
Purpose: Prevents loops in multicast traffic.
IT Example:
- A network monitoring tool receives alerts from a multicast source. If a router doesn’t know the correct path back to the source, it drops the packet to avoid duplicate alerts flooding the network.
3. Protocol Independent Multicast (PIM)
PIM is the main protocol used to route multicast traffic between routers.
Why it’s called “Protocol Independent”:
- It doesn’t depend on a specific unicast routing protocol.
- It uses whatever unicast routing table exists (OSPF, EIGRP, etc.) to build multicast paths.
PIM Modes
- PIM Dense Mode (PIM-DM)
- Floods multicast traffic everywhere first, then prunes areas with no receivers.
- Rarely used now because it can flood large networks.
- PIM Sparse Mode (PIM-SM)
- Only sends traffic to networks that subscribe to it.
- More efficient for large networks.
- Uses a Rendezvous Point (RP), a central router that acts like a meeting point for multicast sources and receivers.
Exam Tip:
- PIM-SM is most commonly tested. Know RP concept, Join/Prune messages, and how it works with IGMP.
4. Internet Group Management Protocol (IGMP)
IGMP allows hosts (devices) to tell routers which multicast groups they want to join.
- IGMPv2
- Supports join/leave messages.
- Routers know when to stop sending multicast traffic if no hosts want it.
- IGMPv3
- Supports source-specific multicast (SSM).
- Hosts can request traffic only from specific sources, making multicast more efficient and secure.
IT Example:
- A network monitoring dashboard wants logs from a specific server. It uses IGMPv3 to subscribe only to that server’s multicast address.
5. How Multicast Works Together
- Host joins a multicast group using IGMP (v2 or v3).
- Router learns about group membership from IGMP messages.
- PIM creates paths from the source to all routers with interested hosts.
- RPF check ensures loops don’t happen.
Flow Example:
- Source server sends multicast updates → Router performs RPF check → Router forwards packet using PIM paths → Receivers subscribed via IGMP get the data.
6. Exam Tips for CCNP
- Remember key protocols and roles:
- IGMP → hosts subscribe to multicast groups.
- PIM → routers deliver multicast traffic efficiently.
- RPF → ensures multicast traffic takes correct paths.
- Know IGMPv2 vs IGMPv3 differences (leave messages, source-specific).
- Understand PIM Dense vs Sparse Mode and Rendezvous Point (RP) concept.
- Be ready to trace multicast flow from source to receivers using RPF and PIM logic.
7. Summary Table (Easy Reference)
| Component | Role | Key Info |
|---|---|---|
| RPF Check | Safety check | Drops multicast if packet arrives on wrong interface |
| PIM | Routing multicast between routers | PIM-DM (flood/prune), PIM-SM (RP-based, efficient) |
| IGMPv2 | Hosts join/leave multicast groups | Simple join/leave messages |
| IGMPv3 | Source-specific multicast | Hosts can choose specific source to receive from |
This covers all the key concepts for CCNP 350-401 exam under 3.4.d. It’s explained simply, without unnecessary real-world analogies, focusing on IT network examples.
