User's Manual

Configuration Guide of XGS-5240-Series
42-6
224.0.0.10 IGRP Router
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 use
s 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, t
hus Multicast MAC address is used. Multicast MAC address is corresponding to Multica
st IP address. It is prescribed in IANA (Internet Assigned Number Authority) that the hi
gher 25 bits in Multicast MAC address is 0x01005e, and the lower 23bits in MAC addr
ess 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.
42.1.3 IP Multicast Packet Transmission
In Multicast mode, the source host sends packets to the host group indicated by th
e Multicast group address in the destination address field of IP data packet. Unlike Uni
cast 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 procedur
e is more complicated than Unicast transmission procedure.
In order to guarantee that all Multicast packets get to the router via the shortest p
ath, 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) che
ck. Multicast router makes use of the impressed packet source address to query Unica
st Router Table or independent Multicast Router Table to determine if the packet ingres
s 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 Multi
cast Data Packets; if Shared Tree is used, then the source address is the address of t
he root of the Shared-Tree. When Multicast data packet gets to the router, if RPF chec
k passes, then the data packet is forwarded according to Multicast forward item, and th