Ip Multicast Packet Transmission - Planet WGSW-52040 Configuration Manual

224.0.0.11 Active Agent
224.0.0.12 DHCP Server/Relay Agent
224.0.0.13 All PIM Routers
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
224.0.0.22 IGMP
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.

38.1.3 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 impressed 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 else wise.
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