The value of vRtrPimNgIfInstantPruneEcho specifies whether PIM
will send an instant Prune Echo when the upstream router enters
Prune-Pending state on this interface. A Prune Echo is simply a Prune
message sent by the upstream router with its own address in the
Upstream Neighbor Address field. Due to this, all downstream routers
will see the Prune message and they can send a join override if
interested in the group.
This is not only useful on NMBA networks, but also can be used on a LAN
enviornment to improve reliability.