The topology as follow :
Multicast source -----AR28-----NE40-----computer
all interface we configured PIM DM ,and also configured "IGMP enable" between NE40 and computer .But we can not receive Multicast packets . also have no multicast routing table .
After checked the configuration, we found that :
1 : there is no acl in NE40.
2 : the IPTV user can receive IPTV program throng DSLAM which directly connect to AR-28.
3 : after captured the packets which came from AR-28, we found that the TTL was 1, so it's the reason .after our NE40 processed ,the TTL will became 0,and NE40 will discard the packets ,so we can not receive .
After change the ttl in the multicast source , problem solved.
Possible reasons :
1. Used ACL to limit the multicast IP address or multicast source Ip address.
2. Problem happened in AR-28.
3. Capture packet in NE40 to analyse.