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System Description
H3C S3600 Series Ethernet Switches
Chapter 3 Software Features
3-7
hearing an IGMP Leave message, it removes the host from the multicast table. By
continuously listening to IGMP packets, the switch creates and maintains a L2 MAC
multicast address table and based on which forwards the multicast packets sent from
the upstream router.
3.4.2 IGMP
The Internet group management protocol (IGMP) runs between hosts and multicast
routers for tracing and learning the membership of the hosts. A multicast router learns
whether there is a multicast member on a subnet connected to it by periodically
sending IGMP Host-Query messages. A host sends IGMP Report messages for joining
a multicast group. The S3600 series support both IGMPv1 and IGMPv2.
3.4.3 PIM-DM
The protocol-independent multicast (PIM) runs between multicast routers. Using PIM, a
multicast router traces and learns which multicast packets are to be forwarded to other
routers, and then transmits them to the LANs connected to the multicast routers.
The protocol independent multicast-dense mode (PIM-DM) is applied to a multicast
environment where multicast group members are dense.
In PIM-DM, a router assumes
that all other routers agree to forward multicast packets for multicast groups.
If the
router receives a multicast packet but has no directly connected multicast group
member or PIM neighbor, it sends a Prune message back to the multicast source.
The
subsequent multicast packets are not to be transmitted to this router.
By using this
flood-prune mechanism, IM-DM creates a multicast distribution tree with the multicast
source as the root.
3.4.4 PIM-SM
Different from PIM-DM, the protocol independent multicast-sparse mode (PIM-SM)
applies to a multicast environment where multicast group members are sparse.
In
PIM-SM, a router assumes that none of other routers agree to forward multicast
packets for multicast groups, unless they declare so. If a host wants to join a multicast
group, the multicast router that is directly connected to it sends a PIM Join message to
the rendezvous point (RP) where the host is registered.
The RP then forwards the Join
message to the multicast source. In PIM-SM, the multicast packet is forwarded along a
shared distribution tree.