If the network FDV is large or the sources are not synchronized, switching from I-PMSI to S-PMSI can happen at a different time on the primary and backup UMHs. This can cause asymmetric traffic on the I-PMSI and S-PMSI, resulting in a switch from the active UMH. The <S,G> traffic can arrive for the I-PMSI from the backup UMH and for S-PMSI from the active UMH, which causes temporary duplicate traffic until both UMHs switch to S-PMSI.
Multistream S-PMSI provides a solution for this case by mapping an <S,G> to an S-PMSI. The <S,G> is locked to the multistream S-PMSI, which is always configured and never torn down, even if the traffic goes down to 0, so the multistream S-PMSI is not susceptible to S-PMSI traffic drops.
The number of <S,G>s must be less than, or equal to, the maximum-p2mp-spmsi value configured under the MVPN selective provider tunnel. Otherwise, different <S,G>s can switch to the S-PMSI and when the S-PMSI limit is exhausted, the 2 UMHs become out of sync.