The value of vRtrPimNgIfGrpSrcAssertState indicates the ASSERT state
for this entry. Where multiple PIM routers peer over a shared LAN it
is possible for more than one upstream router to have valid forwarding
state for a packet, which can lead to packet duplication. PIM does not
attempt to prevent this from occurring. Instead it detects when this
has happened and elects a single forwarder amongst the upstream
routers to prevent further duplication. This election is performed
using PIM Assert messages. Assert messages are also received by
downstream routers on the LAN, and these cause subsequent Join or
Prune messages to be sent to the upstream router that won the Assert.