The value of tmnxPtpPeerLastRxPortId along with
tmnxPtpPeerLastRxPortEncapType and tmnxPtpPeerLastRxPortEncapValue
indicates the last port used to receive PTP packets from the PTP peer.
If no packets have been received, tmnxPtpPeerLastRxPortId contains the
'invalid portid' value of 0x1e000000.