Huawei Quidway S3900 Series Operation Manual page 419

Operation Manual – Multicast Protocol
Quidway S3900 Series Ethernet Switches-Release 1510
will encapsulate the received packet into a registration packet and send it to the
corresponding RP in unicast form, as shown in Figure 7-6:
Source
Server
Figure 7-6 Diagram for SPT building in PIM-SM
When RP receives the registration information from S, it will decapsulate the
registration information and forward the multicast information to the receiver along RPT,
and on the other hand, it will send (S, G) join messages to S hop by hop. The passed
routers constitute a branch of SPT. The multicast source S is the root of SPT and RP is
the destination of RP.
The multicast information sent by the multicast source S will reach RP along the built
SPT, and then RP will forward the multicast information along the built RPT.
VI. Switching RPT to SPT
When the multicast router nearest to the receiver detects that the rate of the multicast
packet from RP to the multicast group G exceeds the threshold value, it will send (S, G)
join messages to the upper-layer router of the multicast source S. The join message
reaches the router nearest to the multicast source (namely, the first hop router) hop by
hop and all the passed routers have the (S, G) entry. As a result, a branch of SPT is
built.
Then, the last hop router sends Prune message with the RP bit to RP hop by hop.
When RP receives the message, it will reversely forward the Prune message to the
multicast source. Thus, the multicast information stream is switched from RPT to SPT.
After the switch from RPT to SPT, the multicast information will be sent from the
multicast source S to the receiver directly. Through the switching from RPT to SPT,
PIM-SM can build SPT in a more economical way than PIM-DM.
The related threshold values are not set on S3900 series Ethernet switches. When the
switch receives multicast data forwarded along RPT, it will update the input interface
automatically and sends Prune messages to RP.
RP
Join
Multicast
Join
Register
packets
Join
SPT
Register
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7-9
Chapter 7 PIM Configuration
User A
Receiver
User B
User C
Receiver
User D
Receiver
User E