Multicast is a communication method in IP networks, often translated from the original English term "Multicast." It's one of three main communication approaches, alongside broadcast and unicast. In industrial fieldbus and Ethernet environments, multicast is widely used to allow multiple slave devices connected to a master station to receive real-time data simultaneously. This ensures that all devices get the same information at roughly the same time. Multicast works by using a special address, known as a multicast address, which can be assigned to multiple destinations. Instead of sending the data separately for each device, it's transmitted once on each network link, and copied when the network branches out. This makes it more efficient than traditional unicast or broadcast methods. To manage these groups of devices, several protocols are used, such as IGMP (Internet Group Management Protocol), PIM (Protocol Independent Multicast), DVMRP (Distance Vector Multicast Routing Protocol), and others. Among them, IGMP is the most commonly supported protocol in industrial Ethernet switches. IGMP allows hosts to join or leave multicast groups, and it helps network devices like switches or routers forward data only to those who need it. This is especially important in Layer 2 switches, which don't support Layer 3 features like full multicast routing. Instead, they use IGMP Snooping to monitor traffic and optimize how multicast packets are delivered. When a host sends an IGMP report to join a group, the switch updates its forwarding table to send the data only to the relevant ports. If the host leaves the group, the port is removed from the list. This prevents unnecessary traffic from flooding the network and improves performance in industrial settings where efficiency is critical. In summary, multicast is a powerful tool for real-time communication in industrial networks, and understanding how protocols like IGMP work is essential for optimizing network performance and reliability.
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