For example, suppose that you run PPP over a RS-232 port. This would use two entries in the ifTable. Let's suppose that entry number 123 is for the PPP "interface" and entry number 987 is for the RS-232 port. So, ifSpecific.123 would contain the ppp OBJECT IDENTIFIER, pppLinkStatusPhysicalIndex.123 would contain 987, and ifSpecific.987 would contain the rs_232 OBJECT IDENTIFIER (or whatever it is).