10.2 Multicast Routing Explained
Key Concepts of Multicast Routing
Multicast Routing is a technique used to efficiently send data to multiple recipients simultaneously. Key concepts include:
- Multicast Group: A group of devices that receive the same multicast traffic.
- IGMP (Internet Group Management Protocol): A protocol used by hosts and routers to manage multicast group memberships.
- PIM (Protocol Independent Multicast): A multicast routing protocol that can work with various unicast routing protocols.
- RP (Rendezvous Point): A central point where multicast sources and receivers register.
- SPT (Shortest Path Tree): A tree that represents the shortest path from the source to all receivers.
- RPT (Shared Tree): A tree that uses the RP as the central point for multicast traffic distribution.
Multicast Group
A Multicast Group is a group of devices that receive the same multicast traffic. Devices join a multicast group to receive data sent to that group. Each multicast group is identified by a unique IP address in the range 224.0.0.0 to 239.255.255.255.
Example: A video conferencing application might use a multicast group address 239.1.1.1 to send video streams to all participants in the conference.
IGMP (Internet Group Management Protocol)
IGMP is a protocol used by hosts and routers to manage multicast group memberships. Hosts use IGMP to inform their local router that they want to join or leave a multicast group. Routers use IGMP to keep track of which groups are active on their connected networks.
Example: When a user starts a video stream on their device, the device sends an IGMP join message to the local router, indicating that it wants to receive traffic sent to the multicast group address 239.1.1.1.
PIM (Protocol Independent Multicast)
PIM is a multicast routing protocol that can work with various unicast routing protocols. It supports two modes: Sparse Mode (PIM-SM) and Dense Mode (PIM-DM). PIM-SM is commonly used in large networks, while PIM-DM is used in smaller, densely connected networks.
Example: In a corporate network, PIM-SM can be configured on MikroTik routers to manage multicast traffic efficiently. The routers use PIM to build multicast distribution trees based on the network topology and group memberships.
RP (Rendezvous Point)
A Rendezvous Point (RP) is a central point where multicast sources and receivers register. In PIM-SM, the RP acts as a meeting point for multicast traffic. Sources send their traffic to the RP, which then forwards it to all receivers in the group.
Example: In a large campus network, a MikroTik router can be configured as the RP. All multicast sources send their traffic to this RP, which then distributes the traffic to the appropriate receivers based on IGMP reports.
SPT (Shortest Path Tree)
A Shortest Path Tree (SPT) is a tree that represents the shortest path from the multicast source to all receivers. In PIM-SM, once a receiver requests data from a specific source, the network builds an SPT to optimize the delivery of multicast traffic.
Example: When a user in a remote office requests a video stream from a source in the headquarters, the network builds an SPT to ensure that the video traffic takes the shortest path from the source to the receiver, minimizing latency and network load.
RPT (Shared Tree)
A Shared Tree (RPT) uses the RP as the central point for multicast traffic distribution. In PIM-SM, the RPT is initially used to deliver multicast traffic from sources to receivers. Once a receiver requests data from a specific source, the network switches to an SPT for optimized delivery.
Example: In a distributed network, the RPT is used initially to distribute multicast traffic from various sources to the RP. When a receiver requests data from a specific source, the network switches to an SPT to ensure efficient delivery of the traffic.