H3C Technologies H3C S12500 Series Switches - Manual (Page 202)

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Figure 55 Where MSDP peers are in the network
As shown in Figure 55 , an MSDP peer can be created on any PIM-SM router. MSDP peers created on
PIM-SM routers that assume different roles function differently.
1. MSDP peers on RPs include the following types:
{
Source-side MSDP peer—The MSDP peer nearest to the multicast source (Source), typically the
source-side RP, like RP 1. The source-side RP creates SA messages and sends the messages to its
remote MSDP peer to notify the MSDP peer of the locally registered multicast source information.
A source-side MSDP peer must be created on the source-side RP. Otherwise it will not be able
to advertise the multicast source information out of the PIM-SM domain.
{
Receiver-side MSDP peer—The MSDP peer nearest to the receivers, typically the receiver-side RP,
like RP 3. After receiving an SA message, the receiver-side MSDP peer resolves the multicast
source information carried in the message and joins the SPT rooted at the source across the
PIM-SM domain. When multicast data from the multicast source arrives, the receiver-side MSDP
peer forwards the data to the receivers along the RPT.
{
Intermediate MSDP peer—An MSDP peer with multicast remote MSDP peers, like RP 2. An
intermediate MSDP peer forwards SA messages received from one remote MSDP peer to other
remote MSDP peers, functioning as a relay of multicast source information.
2. MSDP peers created on common PIM-SM routers (other than RPs)
Router A and Router B are MSDP peers on common multicast routers. Such MSDP peers just
forward received SA messages.
In a PIM-SM network running the BSR mechanism, the RP is dynamically elected from C-RPs. To enhance
network robustness, a PIM-SM network typically has more than one C-RP. As the RP election result is
unpredictable, MSDP peering relationship must be built among all C-RPs so that the winner C-RP is
always on the "MSDP interconnection map", and loser C-RPs will assume the role of common PIM-SM
routers on the "MSDP interconnection map."
Inter-domain multicast delivery through MSDP
As shown in Figure 56 , an active source (Source) exists in the domain PIM-SM 1, and RP 1 has learned
the existence of Source through multicast source registration. If RPs in PIM-SM 2 and PIM-SM 3 also seek
the location of Source so that receiver hosts can receive multicast traffic that originated from it, MSDP
peering relationship must be established between RP 1 and RP 3 and between RP 3 and RP 2.