The value of tmnxPtpPeerLastTxPortId along with
tmnxPtpPeerLastTxPortEncapType and tmnxPtpPeerLastTxPortEncapValue
indicates the last port used to transmit PTP packets to the PTP peer.
If no packets have been transmitted, tmnxPtpPeerLastTxPortId contains
the 'invalid portid' value of 0x1e000000.