Ip Multicast Packet Forwarding - 3Com 7700 Configuration Manual

IP Multicast Packet
Forwarding
resources related (such as bandwidth and CPU of routers) are consumed. In
order to decrease the consumption of these precious network resources,
branches that do not have members send Prune messages toward the source
to reduce the unwanted/unnecessary traffic. To enable the receivers to receive
multicast data streams, the pruned branches can be restored periodically to a
forwarding state. To reduce latency time, the PIM dense mode uses the prune
mechanism to actively restore multicast packet forwarding. The periodical flood
and prune are characteristics of PIM dense mode. Generally, the 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)
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Dense mode uses the flood-prune technology is not applicable for WAN. In
WAN, multicast receivers are sparse and therefore the sparse mode is used. In
sparse mode, hosts need not 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 into the network and is
virtual place for data exchange) corresponding to the group for receiving the
multicast data traffic from the specified group. The join message passes routers
and finally reaches the root, i.e., the RP. The join message 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, PIM sparse mode sends join messages to branches periodically
to maintain the multicast distribution tree.
To send data to the specified address, senders 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 repeated automatically until the packets reach the destination.
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. The multicast model must forward multicast packets to multiple
external interfaces so that packets can be forwarded to all receivers.
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). This check mechanism is the basis for most multicast routing protocols ,
which is known as RPF (Reverse Path Forwarding) check. A multicast router
uses the source address from the multicast packet to query the unicast routing
table or the independent multicast routing table to determine the incoming
interface at which the packet arrives. 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 of the shared tree. When
a multicast packet arrives at the router, if RPF check succeeds, the packet will
be forwarded according to the multicast forwarding entry. Otherwise, the
packet will be dropped.
IP Multicast Overview 113