Protocol:
The multicast routing protocol which created this entry. The
possibilities are PIM DM and PIM SM.
Flags:
The value displayed in this field is valid if the multicast
routing protocol running is PIM SM. The possible values are
RPT or SPT. For other protocols an "------" is displayed.
Detail:
Displays the detailed information of the mroute entries.
Outgoing Interface:
Displays the outgoing interfaces on which multicast packets
for this source/group are forwarded.
11.2
IGMP
Brief Introduction of IGMP
IGMP stands for Internet Group Management Protocol. It is responsible for the management of
IP multicast members in IPv4, and is used to establish and maintain the multicast member
relationships between the IP host and its directly neighboring multicast routers.
So far, there are three IGMP versions:
IGMPv1(defined in RFC 1112)
IGMPv2(defined in RFC 2236)
IGMPv3(defined in RFC 3376)
All IGMP versions support ASM model, and IGMPv3 can be directly applied in SSM model.
IGMPv1 Work Mechanism
IGMPv1 is mainly based on the query-and-response mechanism to manage the multicast group
members.
When there are multiple multicast routers in the subnet, all of them can receive IGMP
membership report message. A specific router needs to be chosen from these routers through
the querier election mechanism, and it will works as the querier to send IGMP query message.
In IGMPv1, the DR (Designated Router) is elected according to the multicast routing protocol
(such as PIM) as the exclusive IGMP querier to forward the multicast information.
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