This field records the port number reported by the device at the far end of this VLAN-on-port record's physical link. For one VLAN as it is provisioned on a single, non-bundled Ethernet uplink port on the fabric interconnect, comparing local and peer port numbers lets UCS Manager render an accurate topology diagram, and a peer port number that keeps changing between polls can itself be a symptom of a flapping or misbehaving link partner. A stable link here would report the same peer port number, say port 39, on every poll rather than jumping around.