Ip Multicast Packet Forwarding - Huawei Quidway S3500 Series Operation Manual

Operation Manual - Multicast
Quidway S3500 Series Ethernet Switches
forwarding path in dense mode is a "source tree" rooted at the source with multicast
members as the branches. Since the source tree uses the shortest path from the
multicast source and the receiver, it is also called the shortest path tree (SPT).
PIM-SM (Protocol-Independent Multicast Sparse Mode, PIM-SM)
Dense mode uses the flood-prune technology, which is not applicable for WAN. In WAN,
multicast receivers are sparse and the sparse mode are mostly used. In sparse mode,
all hosts do not need to receive multicast packets unless there is an explicit request for
the packets by default. A multicast router must send a join message to the RP
(Rendezvous Point, which needs to be built up in the network and is the virtual place for
data exchange) corresponding to the group to receive the multicast data traffic from the
specified group. The join message passes routers and finally reaches the root, i.e., the
RP. The path the join message passed becomes a branch of the shared tree. In PIM
sparse mode, multicast packets are sent to the RP first and then are forwarded along
the shared tree rooted at the RP and with members as the branches. To prevent the
branches of the shared tree from being deleted for they not updated, PIM sparse mode
sends join messages to branches periodically to maintain the multicast distribution tree.
To send data to the specified address, senders should register with the RP first before
forwarding data to the RP. When the data reaches the RP, the multicast packets are
replicated and sent to receivers along the path of the distribution tree. Replicate only
happens at the branches of the distribution tree. This process can be automatically
repeated until the packets reach the destination.

1.4 IP Multicast Packet Forwarding

In the multicast model, the source host sends information to the host group represented
by the multicast group address within the destination address fields of the IP packets.
Different from the unicast model, the multicast model must forward the multicast
packets to multiple external interfaces so that the packets can be sent to all receivers.
Therefore, the multicast forwarding process is much more complex than the unicast
forwarding process.
RPF (Reverse Path Forwarding)
To ensure that a multicast packet reaches the router along the shortest path, the
multicast must depend on the unicast routing table or a unicast routing table
independently provided for multicast (such as the MBGP multicast routing table) to
check the receiving interface of multicast packets. This check mechanism is the basis
for most multicast routing protocols performing multicast forwarding, which is known as
RPF (Reverse Path Forwarding) check. A multicast router uses the source address at
which the multicast packet arrives to query the unicast routing table or the independent
multicast routing table so as to determine that the incoming interface at which the
packet arrives is on the shortest path from the receiver to the source address. If a
source tree is used, the source address is the address of the source host sending the
multicast packet. If a shared tree is used, the source address is the address of the root
Huawei Technologies Proprietary
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Chapter 1 IP Multicast Overview