Thanks Alex for your remark.
I update the configuration. PIM is now enable between vR2 and vR4 as shown below
root@ISP-A:vR4> show pim interfaces
Stat = Status, V = Version, NbrCnt = Neighbor Count,
S = Sparse, D = Dense, B = Bidirectional,
DR = Designated Router, P2P = Point-to-point link,
Active = Bidirectional is active, NotCap = Not Bidirectional Capable
Name Stat Mode IP V State NbrCnt JoinCnt(sg/*g) DR address
lt-0/0/0.45 Up S 4 2 NotDR,NotCap 1 0/0 10.0.10.6
lt-0/0/0.46 Up S 4 2 DR,NotCap 1 0/0 10.0.10.2
lt-0/0/0.92 Up S 4 2 DR,NotCap 1 0/0 172.17.20.2
pd-0/0/0.55297 Up S 4 2 P2P,NotCap 0 0/0
root@ISP-A:vR4> set cli logical-system vR2
Logical system: vR2
root@ISP-A:vR2> show pim interfaces
Stat = Status, V = Version, NbrCnt = Neighbor Count,
S = Sparse, D = Dense, B = Bidirectional,
DR = Designated Router, P2P = Point-to-point link,
Active = Bidirectional is active, NotCap = Not Bidirectional Capable
Name Stat Mode IP V State NbrCnt JoinCnt(sg/*g) DR address
lt-0/0/0.21 Up S 4 2 DR,NotCap 1 2/0 10.0.5.2
lt-0/0/0.23 Up S 4 2 NotDR,NotCap 1 0/0 10.0.5.10
lt-0/0/0.29 Up S 4 2 NotDR,NotCap 1 0/0 172.17.20.2
pd-0/0/0.51202 Up S 4 2 P2P,NotCap 0 0/0
But unfortunately I still have the flag reject for SRC-A on vR4
root@ISP-A:vR4> show msdp source-active
Global active source limit exceeded: 0
Global active source limit maximum: 25000
Global active source limit threshold: 24000
Global active source limit log-warning: 100
Global active source limit log interval: 0
Group address Source address Peer address Originator Flags
224.1.1.20 10.0.30.1 172.17.20.1 192.168.1.2 Reject