3Com Switch 8800 Advanced Software V5 Configuration Guide

612 CHAPTER 43: MSDP CONFIGURATION
and sends the register message to RP 1. Then, RP 1 gets aware of the information
related to the multicast source.
2 As the source-side RP, RP 1 creates SA messages and periodically sends the SA
messages to its MSDP peer. An SA message contains the source address (S), the
multicast group address (G), and the address of the RP which has created this SA
message (namely RP 1).
3 On MSDP peers, each SA message is subject to a reverse path forwarding (RPF)
check and multicast policy-based filtering, so that only SA messages that have
arrived along the correct path and passed the filtering are received and forwarded.
This avoids delivery loops of SA messages. In addition, you can configure MSDP
peers into an MSDP mesh group so as to avoid flooding of SA messages between
MSDP peers.
4 SA messages are forwarded from one MSDP peer to another, and finally the
information of the multicast source traverses all PIM-SM domains with MSDP
peers (PIM-SM 2 and PIM-SM 3 in this example).
5 Upon receiving the SA message create by RP 1, RP 2 in PIM-SM 2 checks whether
there are any receivers for the multicast group in the domain.
■ If so, the RPT for the multicast group G is maintained between RP 2 and the
receivers. RP 2 creates an (S, G) entry, and sends an (S, G) join message hop by
hop towards DR 1 at the multicast source side, so that it can directly join the
SPT rooted at the source over other PIM-SM domains. Then, the multicast data
can flow along the SPT to RP 2 and is forwarded by RP 2 to the receivers along
the RPT. Upon receiving the multicast traffic, the DR at the receiver side (DR 2)
decides whether to initiate an RPT-to-SPT switchover process.
■ If no receivers for the group exist in the domain, RP 2 does dot create an (S, G)
entry and does join the SPT rooted at the source.
n
■ An MSDP mesh group refers to a group of MSDP peers that have an MSDP
peering relationship among one another and share the same group name.
■ When using MSDP for inter-domain multicasting, once an RP receives
information form a multicast source, it no longer relies on RPs in other PIM-SM
domains. The receivers can override the RPs in other domains and directly join
the multicast source based SPT.
RPF check rules for SA messages
As shown in Figure 188, there are five autonomous systems in the network, AS 1
through AS 5, with IGP enabled on routers within each AS and EBGP as the
interoperation protocol among different ASs. Each AS contains at least one
PIM-SM domain and each PIM-SM domain contains one ore more RPs. MSDP
peering relationships have been established among different RPs. RP 3, RP 4 and
RP 5 are in an MSDP mesh group. On RP 7, RP 6 is configured as its static RPF peer.
n
If only one MSDP peer exists in a PIM-SM domain, this PIM-SM domain is also
called a stub domain. For example, AS 4 in
Figure 188 is a stub domain. The MSDP
peer in a stub domain can have multiple remote MSDP peers at the same time.
You can configure one or more remote MSDP peers as static RPF peers. When an
RP receives an SA message from a static RPF peer, the RP accepts the SA message
and forwards it to other peers without performing an RPF check.