160
, an MSDP peer can be created on any PIM-SM router. MSDP peers created
on PIM-SM routers that assume different roles function differently.
•
MSDP peers created on RPs:
Source-side MSDP peer
—MSDP peer closest to the multicast source, such as RP 1. The
source-side RP creates and sends SA 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 cannot
advertise the multicast source information out of the PIM-SM domain.
Receiver-side MSDP peer
—MSDP peer closest to the receivers, typically the receiver-side
RP, such as RP 3. After receiving an SA message, the receiver-side MSDP peer resolves
the multicast source information carried in the message. Then, it joins the SPT rooted at the
multicast source across the PIM-SM domains. 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
—MSDP peer with multiple remote MSDP peers, such as RP 2.
An intermediate MSDP peer forwards SA messages received from one remote MSDP peer
to other remote MSDP peers. It acts as a relay for forwarding multicast source information.
•
MSDP peers created on PIM-SM routers that are not RPs:
Router A and Router B are MSDP peers on multicast routers that are not RPs. Such MSDP
peers only forward SA messages.
In a PIM-SM network using the BSR mechanism, the RP is dynamically elected from C-RPs. A
PIM-SM network typically has multiple C-RPs to ensure network robustness. Because the RP
election result is unpredictable, MSDP peering relationships must be built between all C-RPs to
always keep the winning C-RP on the MSDP interconnection map. Losing C-RPs assume the role of
common PIM-SM routers on this map.
Inter-domain multicast delivery through MSDP
As shown in
, an active source (Source) exists in the domain PIM-SM 1, and RP 1 has
learned the existence of Source through multicast source registration. RPs in PIM-SM 2 and PIM-SM
3 also seek the location of Source so that multicast traffic from Source can be sent to their receivers.
MSDP peering relationships must be established between RP 1 and RP 3 and between RP 3 and RP
2.