dedicated host, but rather a group, i.e. during the routing of multicast packets, the decisive
factor is whether a recipient is in a logged-in subnet.
In the local network, all hosts are required to accept all multicast packets. For Ethernet or
FDD, this is based on MAC mapping, where the group address is encoded into the destina-
tion MAC address. For routing between several networks, the routers first need to make
themselves known to all potential recipients in the subnet. This is achieved by means of
Membership Management protocols such as IGMP for IPv4 and MLP for IPv6.
Membership Management protocol
In IPv4, IGMP (Internet Group Management Protocol) is a protocol that hosts can use to
provide the router with multicast membership information. IP addresses of the class D ad-
dress range are used for addressing. An IP address in this class represents a group. A
sender (e.g. Internet radio) sends data to this group. The addresses (IP) of the various
senders within a group are called the source (addresses). Several senders (with different
IP addresses) can therefore transmit to the same multicast group, leading to a 1-to-n rela-
tionship between groups and source addresses. This information is forwarded to the router
by means of reports. In the case of incoming multicast data traffic, a router can use this in-
formation to decide whether a host in its subnet wants to receive it. Your device supports
the current version IGMP V3, which is upwardly compatible, which means that both V3 and
V1/V2 hosts can be managed.
Your device supports the following multicast mechanisms:
• Forwarding: This relates to static forwarding, i.e. incoming data traffic for a group is
passed in all cases. This is a useful option if multicast data traffic is to be permanently
passed.
• IGMP: IGMP is used to gather information about the potential recipients in a subnet. In
the case of a hop, incoming multicast data traffic can thus be selected.
Tip
With multicast, the focus is on excluding data traffic from unwanted multicast groups.
Note that if forwarding is combined with IGMP, the packets can be forwarded to the
groups specified in the forwarding request.
16.1 General
16 Multicast
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