R3#sh run | i ip pim sen
ip pim send-rp-announce FastEthernet0/1 scope 1 group-list 1 interval 10
R3#
R3#sh run int s 1/2
Building configuration...

Current configuration : 92 bytes
!
interface Serial1/2
 ip address 180.40.7.3 255.255.255.224
 ip pim sparse-dense-mode  <-------------on all routers
end