Qtech QSW-2900-24T-AC [89/209] Figure 1 5 mapping relationship between multicast ip address and multicast mac address

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different multicast domains, so that the same multicast address can be used in different

multicast domains without causing collisions.

b) Ethernet multicast MAC address

When a unicast IP packet is transported in an Ethernet network, the destination MAC address is the MAC

address of the receiver. When a multicast packet is transported in an Ethernet network, a multicast MAC address is

used as the destination address because the destination is a group with an uncertain number of members.

As stipulated by IANA, the high-order 24 bits of a multicast MAC address are 0x01005e, while the low-order

23 bits of a MAC address are the low-order 23 bits of the multicast IP address. Figure 1-5 describes the mapping

relationship:

Figure 1-5 Mapping relationship between multicast IP address and multicast MAC address

The high-order four bits of the IP multicast address are 1110, representing the multicast ID. Only 23 bits of the

remaining 28 bits are mapped to a MAC address. Thus, five bits of the multicast IP address are lost. As a result, 32 IP

multicast addresses are mapped to the same MAC address.

5.2 GMRP Overview

GMRP (GARP Multicast Registration Protocol), based on GARP, is used for maintaining multicast

registration information of the switch. All GMRP-capable switches can receive multicast registration information

from other switches, dynamically update local multicast registration information, and send their own local multicast

registration information to other switches. This information switching mechanism keeps consistency of the multicast

information maintained by every GMRP-supporting device in the same switching network.

A host sends a GMRP Join message, if it is interested in joining a multicast group. After receiving the message,

the switch adds the port on which the message was received to the multicast group, and broadcasts the message

throughout the VLAN where the receiving port resides. In this way, the multicast source in the VLAN gets aware of

the existence of the multicast group member. When the multicast source sends multicast packets to a group, the

switch only forwards the packets to ports connected to the members of that group, thereby implementing Layer 2

multicast in the VLAN.