The value of tmnxOamTrProbeHistoryAddressType indicates the type of
binding address information returned in response to a test.
When the test has the tmnxOamTrCtlTestMode value 'ethCfmLinkTrace',
'unknown' is returned.
For 'macTraceRoute', three types of address are applicable - 'sapId',
'localCpu', and 'sdpId'. Address type of 'sdpId' is a special case
used specifically to indicate that the probe was traversing a b-vpls
node in a PBB network (but where destination MAC is not necessarily
known to the b-vpls node).