In topologies where VOSS platforms are interoperating with ERS or VSP 9000 Series platforms, the selection of the RP from multiple candidate RPs may produce different results on VOSS than it does on ERS or VSP 9000 Series. VOSS platforms conform to PIM RFC 4601, while ERS and VSP 9000 Series platforms conform to RFC 2362. RFC 4601 is not backward compatible with RFC 2362 regarding how it defines the selection algorithm for an RP, specifically when there are several candidate RPs for the same group, but with different prefix lengths. Both RFCs have the RP selection mechanism based on a specific hash function, common to all routers in PIM domain, however there are differences in determining the pool of candidate RPs to which the hash function will be applied. In RFC 4601, only the RP of the group range with the longest prefix match for the group range will be chosen to apply the hash function and thus participate in the actual election. In RFC 2362, longest prefix match is not part of the selection criteria, and therefore ERS/VSP 9000 Series could potentially choose a different RP, since they will apply the hash function on a different pool of candidate RPs. This would cause inconsistencies in the PIM-SM network.
In order to work around this issue, please define RP group ranges with the same prefix length, such that the next RFC-defined match rule will be applied equally across all platforms in the network.