3Com 7757 Configuration Manual page 505

keyword is optional. If you do not use this keyword, all (S, G) entries will be filtered
out by default, that is, none of the (S, G) entries in the local multicast domain will
be advertised. Before the import-source command is executed, the system will
send all (S, G) entries in the local multicast domain. If the MSDP fails to send the
(S, G) entries of the local multicast domain through SA messages, verify that the
import-source command is configured correctly.
Solution
1 Check the connectivity of the route between the routers. Use the display ip
routing-table command to check that the unicast route between the routers is
correct.
2 Further check that a unicast route exists between two routers that will become
MSDP peers and that the route leads to the two peers.
3 Verify the configuration of the import-source command and the corresponding
ACL to ensure that the ACL rule filters the right (S, G) entries.
Troubleshooting MSDP Configuration 505