The entPhysicalVendorType from the ENTITY-MIB is the best object to use. This will return an OID for each object in the device (including ports). You can then look in the CISCO-ENTITY-VENDORTYPE-OID-MIB to get a description of those objects. For example, a copper 10/100/1000 port will be cevPortBaseTEther, a 100 BaseT port will be cevPortNIC100, and a GBIC gigabit port will be cevPortGigBaseSX.
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