3Com Switch 8800 Advanced Software V5 Configuration Guide

Multicast Architecture 497
■ Permanent group addresses: Multicast addresses reserved by IANA for routing
protocols. Such an address identifies a group of specific network devices (also
known as reserved multicast groups). For detail, see
Table 26. A permanent
group address will never change. There can be any number of, or even 0,
members in a permanent multicast group.
■ Temporary group addresses: Group addresses that are temporarily assigned for
user multicast groups. Once the number of members of a group comes to 0,
the address is released.
Multicast MAC addresses
When a unicast IP packet is transmitted over an Ethernet network, the destination
MAC address is the MAC address of the receiver. When a multicast packet is
transmitted over an Ethernet network, however, a multicast MAC address is used
as the destination address because the packet is directed to a group formed by a
number of receivers, rather than to a specific receiver.
As defined by IANA, the high-order 24 bits of a multicast MAC address are
0x01005e, bit 25 is 0x0, and the low-order 23 bits are the low-order 23 bits of a
multicast IP address. The IPv4-to-MAC mapping relation is shown in
Figure 158.
Tab le 26 Some reserved multicast addresses
Address Description
224.0.0.1 All systems on this subnet, including hosts and
routers
224.0.0.2 All multicast routers on this subnet
224.0.0.3 Unassigned
224.0.0.4 Distance Vector Multicast Routing Protocol (DVMRP)
routers
224.0.0.5 Open Shortest Path First (OSPF) routers
224.0.0.6 OSPF designated routers/backup designated routers
224.0.0.7 Shared Tree (ST) routers
224.0.0.8 ST hosts
224.0.0.9 Routing Information Protocol version 2 (RIPv2)
routers
224.0.0.11 Mobile agents
224.0.0.12 Dynamic Host Configuration Protocol (DHCP) server
/ relay agent
224.0.0.13 All Protocol Independent Multicast (PIM) routers
224.0.0.14 Resource Reservation Protocol (RSVP) encapsulation
224.0.0.15 All Core-Based Tree (CBT) routers
224.0.0.16 Designated Subnetwork Bandwidth Management
(SBM)
224.0.0.17 All SBMs
224.0.0.18 Virtual Router Redundancy Protocol (VRRP)