As an MSDP router learns of new sources either through a PIM-SM Source-Register (SR) message or SA message from its RPF peer, it creates an entry in SA cache (or refreshes the entry if it is already there) and forwards this information to its peers. These entries are refreshed by periodic SA messages received from the MSDP peers. If these entries are not refreshed within six minutes, they will time out. When a PIM-SM RP detects that the source is no longer available it informs MSDP, which in turn removes the SA information from the local database.
Caching makes it easy for local receivers to know immediately about inter-domain multicast sources and to initiate building a source tree towards the source. However, maintaining a cache is heavy both in CPU processing and memory requirements.
NoteOur implementation of MSDP does not support operating with local cache disabled.
As MSDP uses the flood-and-join model to propagate information about sources, there is a restriction that no more than two advertisements per cache entry will be forwarded per advertisement interval. This is helpful in reducing an SA message storm and unnecessarily forwarding them to peers.
By default, the router does not send SA request messages to its MSDP peers when a new member joins a group and wants to receive multicast traffic. The new member simply waits to receive SA messages, which eventually arrive.