The LLDP spec says that if lldpRemPortIdSubtype is
"interface name", then lldpRemPortId should contain
ifName. It doesn't say much about lldpRemPortDesc;
some vendors seem to always leave it empty, some use
the interface description / ifAlias. Our algorithm
for lldp_port is now basically: If we find that
lldpRemPortIdSubtype is "interface name", then use
the lldpRemPortId, else use lldpRemPortDesc if set,
else use lldpRemPortId.
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