Multicast Protocols - H3C S5810 Series Operation Manual

Ethernet switches

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Ethernet multicast MAC addresses

When a unicast IP packet is transmitted over Ethernet, the destination MAC address is the MAC

address of the receiver. When a multicast packet is transmitted over Ethernet, however, the destination

address is a multicast MAC address because the packet is directed to a group formed by a number of

receivers, rather than to one specific receiver.

As defined by IANA, the high-order 24 bits of an IPv4 multicast MAC address are 0x01005E, bit 25 is 0,

and the low-order 23 bits are the low-order 23 bits of a multicast IPv4 address. The IPv4-to-MAC

mapping relation is shown in

Figure 1-4 IPv4-to-MAC address mapping

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.

Multicast Protocols

Shared Tree (ST) routers

Routing Information Protocol version 2 (RIPv2) routers

Mobile agents

Dynamic Host Configuration Protocol (DHCP) server/relay agent

All Protocol Independent Multicast (PIM) routers

Resource Reservation Protocol (RSVP) encapsulation

All Core-Based Tree (CBT) routers

Designated Subnetwork Bandwidth Management (SBM)

Virtual Router Redundancy Protocol (VRRP)

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