The value of tmnxBsxAarpInstPeerSubRefType indicates the
type of the subscriber which the peer of this AARP instance
references with a service type of 'dualHomed(1)'.
The valid values are 'unknown(0)' indicating the lack of a
service reference with type 'dualHomed(1)', 'sap(2)', and
'spokeSdp(3)'.