Accton Technology ES4626 - Manual (Page 613)

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224.0.0.14 RSVP Encapsulation
224.0.0.15 All CBT Routers
224.0.0.16 Specified SBM
224.0.0.17 All SBMS
224.0.0.18 VRRP
When Ethernet transmits Unicast IP messages, the destination MAC address it uses
is the receiver’s MAC address. But in transmitting Multicast packets, the transmission
destination is not a specific receiver any more, but a group with uncertain members, thus
Multicast MAC address is used. Multicast MAC address is corresponding to Multicast IP
address. It is prescribed in IANA (Internet Assigned Number Authority）that the higher 25
bits in Multicast MAC address is 0x01005e, and the lower 23bits in MAC address is the
lower 23bits in Multicast IP address.
Since only 23bits out of the lower 28bits in IP Multicast address are mapped into
MAC address, therefore there are 32 IP Multicast addresses which are mapped into the
same MAC address.
16.1.3 IP Multicast Packet Transmission
In Multicast mode, the source host sends packets to the host group indicated by the
Multicast group address in the destination address field of IP data packet. Unlike Unicast
mode, Multicast data packet must be forwarded to a number of external interfaces to be
sent to all receiver sites in Multicast mode, thus Multicast transmission procedure is more
complicated than Unicast transmission procedure.
In order to guarantee that all Multicast packets get to the router via the shortest path,
the receipt interface of the Multicast packet must be checked in some certain way based
on Unicast router table; this checking mechanism is the basis for most Multicast Routing
Protocol to forward in Multicast mode --- RPF (Reverse Path Forwarding) check.
Multicast router makes use of the ingressed packet source address to query Unicast
Router Table or independent Multicast Router Table to determine if the packet ingress
interface is on the shortest path from receipt site to source address. If shortest path Tree
is used, then the source address is the address of source host which sends Multicast
Data Packets; if Shared Tree is used, then the source address is the address of the root
of the Shared-Tree. When Multicast data packet gets to the router, if RPF check passes,
then the data packet is forwarded according to Multicast forward item, and the data
packet will be discarded elsewise.
16.1.4 IP Multicast Application
IP Multicast technology has effectively solved the problem of sending in single