3Com Switch 4800G 24-Port Configuration Manual page 548

548 C 41: M
HAPTER ULTICAST
Multicast Protocols
n
O
VERVIEW
Figure 166 IPv4-to-MAC address mapping
32-bit IPv4 address
48-bit MAC address
0000 0001 0000 0000
25-bit MAC address prefix
The high-order four bits of a multicast IPv4 address are 1110, indicating that this
address is a multicast address, and only 23 bits of the remaining 28 bits are
mapped to a MAC address, so five bits of the multicast IPv4 address are lost. As a
result, 32 multicast IPv4 addresses map to the same MAC address. Therefore, in
Layer 2 multicast forwarding, a device may receive some multicast data addressed
for other IPv4 multicast groups, and such redundant data needs to be filtered by
the upper layer.
2 IPv6 multicast MAC addresses
The high-order 16 bits of an IPv6 multicast MAC address are 0x3333, and the
low-order 32 bits are the low-order 32 bits of a multicast IPv6 address. Figure 167
shows an example of mapping an IPv6 multicast address, FF1E::F30E:0101, to a
MAC address.
Figure 167 An example of IPv6-to-MAC address mapping
128-bit IPv6 address
F F1 E 0000
Generally, we refer to IP multicast working at the network layer as Layer 3
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multicast and the corresponding multicast protocols as Layer 3 multicast
protocols, which include IGMP/MLD, PIM/IPv6 PIM, and MSDP; we refer to IP
multicast working at the data link layer as Layer 2 multicast and the
corresponding multicast protocols as Layer 2 multicast protocols, which include
IGMP Snooping/MLD Snooping, and multicast VLAN/IPv6 multicast VLAN.
IGMP Snooping, IGMP, multicast VLAN, PIM and MSDP are for IPv4, MLD
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Snooping, MLD, IPv6 multicast VLAN, and IPv6 PIM are for IPv6.
5 bits lost
XXXX X
1110 XXXX XXXX XXXX
0101 1110 0XXX XXXX
0000 0000 0000
48-bit MAC address
XXXX XXXX XXXX XXXX
23 bits
mapped
XXXX XXXX XXXX XXXX
0000 F 30E
32 bits
mapped
3333 F 30E a
16-bit MAC
address prefix
0101
0101