Pim-Sm - Zte ZXR10 5900E Series User Manual

2. The packet copy only occurs in the branch position which
makes the sent packets over network minimum.
A multicast tree is a set, which is composed of a series of in-
put interfaces and output interfaces of routes. It determines only
one forwarding path between both the subnet where the multicast
source lies and the subnets containing group members.
There are two basic methods to construct multicast trees: the
source-based multicast tree and the multicast rendezvous point
tree (RPT).
1. Source-Based Multicast Tree
The source-based multicast tree is also called source shortest
path tree. It constructs a spanning tree toward all receivers for
each source. The spanning tree with the subnet of the source
as a root node extends to the subnet where receivers exist. A
multicast group may have multiple multicast sources.
Each source or each pair (S, G) has a corresponding multicast
tree.
Method to construct the source-based multicast trees is the re-
verse path forwarding (RPF). Each router can find the shortest
path toward the source and the corresponding output inter-
face according to a unicast route. When receiving a multicast
packet, a router checks whether the input interface receiving it
is the output interface of the shortest unicast path toward the
source itself. If so, the router copies and forwards the packet
to other interfaces. If not, the router discards the multicast
packet.
The input interface receiving the multicast packet in the router
is called the father link. The output interface sending the mul-
ticast packet is called the sub-link.
2. Multicast Rendezvous Point Tree
The multicast rendezvous point tree constructs a multicast tree
for each multicast group. This tree is shared by all members.
Namely, the (*, G) use commonly a preceding multicast tree
but not construct a tree for each pair (S, G). Each device want-
ing to receive the multicast packet of the group must explicitly
join the RPT.
The multicast RPT uses a router or a group of routers as the
center of the multicast tree. All source direction receivers send
the packet to the center in a unicast mode, and start to forward
from the center along the multicast RPT tree in a multicast
mode.

PIM-SM

PIM-SM sends multicast packet by using the multicast RPT. A mul-
ticast RPT has a center point that is responsible for sending packets
to the ports of all sources of a multicast group. Send ports of each
source sends packets to the center point along the shortest path
and distribute packets to receiving ports. The group center point
of the PIM-SM is called Rendezvous Point (RP). A network may
have multiple RPs, whereas a multicast group has only one RP.
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Chapter 6 Multicast Routing Configuration
67