Source Active Messages

In addition to keep-alive messages, MSDP peers send each other SA messages. SA messages convey group addresses, source addresses, and RP addresses among MSDP peers so that the RPs in different domains learn active source information from outside. MSDP SA messages are sent out periodically by the originating RP.

MSDP routers cache SA messages they receive from their MSDP peers. They also cache SA messages received from RPs within their own PIM domain. The SA-cache can reduce message storms. Entries in cache time out after 135 seconds and are removed from the cache. SA messages can also be cleared manually.

MSDP uses filters to control SA source information. Inbound filters determine if an SA message is to be accepted or which peer it is to be accepted from. If no SA filters are configured, the MSDP router receives all SA messages from its RPF peers and forwards them to the other MSDP peers. Outbound policy determines which peer the SA messages are sent to and for which source/group to advertise. The default is to advertise any source to any MSDP peer. Extended access control lists are used in SA filters to filter incoming SA messages and to prevent outgoing messages from being forwarded.