Steps to reproduce:

1. Apply "Cisco IOS by SNMP" template to network equipement. 
2. Let "Network interfaces discovery" do its thing. 

Result:

Network interfaces that are down do not report bits received/sent and therefore OIDs does not exist. For example:

snmpwalk -v 2c -c public <host> ifHCInOctets.32
IF-MIB::ifHCInOctets.32 = No Such Instance currently exists at this OID

Therefore item becomes unsupported.

Fix:

Add preprocessing "check for error using regular expression" step on item prototype for "Interface {#IFNAME}({#IFALIAS}): Bits received" and "Interface {#IFNAME}({#IFALIAS}): Bits sent" using expression "(no data was found)" and setting value to 0.