3Com 3C13636 Configuration Manual page 785

3Com Router 3000 Ethernet Family
Configuration Guide
3) The RP in domain 1 decapsulates the packet and forwards it along the RPT to all
the members within the domain. The domain members can choose to take the
path along SPT.
4) The RP in domain 1 generates an SA (Source Active) message for the MSDP
peers (the RPs in PIM-SM domain 2 and domain 3). The SA message contains
multicast source IP address, multicast group address, the RP address as well as
the multicast this RP first receives.
5) If there is any group member in the domain of an MSDP peer (in the figure, it is
PIM-SM domain 3), the RP in this domain sends the multicast data encapsulated
in the SA message to group members along the RPT and the join message to
multicast source.
6) After the reverse forwarding path is created, the multicast source data is sent
directly to the RP in domain 3, which then RP forwards the data along the RPT. In
this case, the last hop router connected with the group member in domain 3 can
choose whether to switch to SPT.
RP&MSDP Peer RP&MSDP Peer
PIM-SM 2 PIM-SM 2
Figure 5-1 MSDP working principles I
As shown in Figure 5-2, RTA, RTB, RTC, RTD, RTE and RTF belong to domain 1,
domain 2 and domain 3 respectively. MSDP peer relationship is established between
them, indicated with bi-directional arrows in the figure. Among them, Mesh Group is
created among RTB, RTC and RTD. The SA message forwarding and RPF check
among these MSDP peers are illustrated as follows:
1) If the SA message is from an MSDP peer that is the RP of the multicast source as
from RTA to RTB, it is received and forwarded to other peers.
2) If the SA message is from an MSDP peer that has only one peer as from RTB to
RTA, it is received.
PIM-SM 1 PIM-SM 1
DR DR
2 2
Register Register
Group Group
3 3
Member Member
RP&MSDP Peer RP&MSDP Peer
4 4
4 4
3Com Corporation
5-2
Chapter 5 MSDP Configuration
1 1
Source Source
5 5
4 4
RP&MSDP Peer RP&MSDP Peer
5 5
PIM-SM 3 PIM-SM 3
Group Group
Member Member