Manual

118
XMS-1024P
a: 14203 Minuteman Drive, Suite 201, Draper, UT 84020-1685 | luxul.com | 801-822-5450
LUX-UG-XMS-1024P Vers: 081314
Figure 8-2 Mapping relationship between Multicast IP Address and Multicast MAC
address
The high-order 4 bits of the IP Multicast address are 1110, identifying the Multicast
group. Only 23 bits of the remaining low-order 28 bits are mapped to a Multicast MAC
address. In this confi guration, 5 bits of the IP Multicast Address are not utilized. As a
result, 32 IP Multicast addresses are mapped to the same MAC address.
Multicast MAC Table
The Switch forwards Multicast packets based on the Multicast MAC Table. As the
transmission of Multicast packets cannot span VLANs, the fi rst part of the Multicast
MAC Table is VLAN ID. Based on which VLAN ID the received Multicast packets have,
the Multicast packets are forwarded to the ports that are a member of the correspond-
ing VLAN. The Multicast MAC Table is not mapped to an Egress Port but a group Port
list. When forwarding a Multicast packet, the Switch looks up the Multicast MAC Table
based on the destination Multicast Address of the Multicast packet. If the correspond-
ing entry cannot be found in the table, the Switch will broadcast the packet to all of
the member ports of the VLAN. If the corresponding entry can be found in the table,
it indicates that the destination Address should be a group Port list, so the Switch will
duplicate the Multicast Data and deliver it each Port in the group. The general format of
the Multicast MAC Table is described in Figure 8-3 below.
VLAN ID
Multicast IP Port
Figure 8-3 Multicast MAC Table
IGMP Snooping
In the Network client devices apply to the nearest Multicast Server to join/leave a
Multicast group by sending IGMP (Internet Group Management Protocol) messages.
When the up-stream device forwards the Multicast Data, the Switch is responsible for
sending them to the client devices. IGMP Snooping is a Multicast control mechanism,
which can be used on the Switch for dynamic registration of devices in the Multicast
group. A Switch running IGMP Snooping manages and controls the Multicast group by
listening for and processing the IGMP messages transmitted between the client
devices and the Multicast Server, this prevents Multicast groups being broadcasted
on the Network.