3Com Switch 7750 Configuration Guide Guide

442 CHAPTER 45: MSDP CONFIGURATION
Figure 107 Identifying the multicast source and receiving multicast data
The complete interoperation process between a multicast source S in the PIM-SM1
domain and receivers in the PIM-SM1 and PIM-SM4 domains is as follows:
1 The multicast source S in the PIM-SM1 domain begins to send data packets;
2 The designated router (DR) connected to the multicast source S encapsulates the
received data in a Register message, and then forwards the message to RP1 in the
PIM-SM1 domain;
3 RP1 in the PIM-SM1 domain decapsulates the Register message, and then
forwards the message to all the members in the domain along the RPT. The
members in the domain can select whether to switch to the SPT;
4 At the same time, RP1 in the PIM-SM1 domain generates an SA message and
sends the message to the corresponding MSDP peers (RPs in the PIM-SM2 and
PIM-SM3 domains). Finally, the SA message is forwarded to the RP in the PIM-SM4
domain. The SA message contains the IP address of the multicast source, the
multicast group address, the address of the RP that has generated the SA
message, and the first multicast data received by the RP in the PIM-SM1 domain;
5 If group members (namely, receivers) exist in the PIM-SM domains where MSDP
peers reside (for example, if group members exist in the PIM-SM4 domain), RP4
decapsulates the multicast data in the SA message and distributes the multicast
data to receivers along the RPT. RP4 also sends a Join message to the multicast
source S at the same time;
6 To avoid SA loop, MSDP peers perform RPF check on the received SA message.
After the RPF path is established, the data from the multicast source S is directly
sent to RP4 in the PIM-SM4 domain. Then, RP4 forwards the data along the RPT
within the domain. Now, the last-hop router connected to group members in the
PIM-SM4 domain selects whether to switch to the SPT.
Forwarding messages between MSDP peers and performing RPF check
To establish an MSDP peering relationship between routers, you have to create
BGP (IBGP or EBGP) or MBGP peers between routers to provide BGP/MBGP routes
for SA messages to travel.
RP1
Source
RP2
RP4
RP3
PIM-SM 1
PIM-SM 2
PIM-SM 3
PIM-SM 4
user
(5)
(5)
DR
(1)
(2)
user
(3)
(4)
(4)
(4)
(4)
(4)
(5)
Flow
MSDP peers