Q. Why do some interfaces display [[Not in ifTable]] in the interface description field?
A.The notation [[Not in ifTable]] means that InterMapper picked up references to certain ifIndex values while scanning the ipAddrTable that don't exist in the ifTable.
This is a normal situation for VLANs. InterMapper first traverses the ipAddrTable which maps IP addresses to ifIndex entries. It then looks for the interface description from the corresponding entries in the ifTable. If the VLAN has an entry in the ipAddrTable, InteMapper will display it as an unnumbered interface. However, that interface will not have an entry in the ifTable.
For example, the ipAddrTable might look like this:
ifIndex 1 --> 192.168.1.1/24
ifIndex 2 --> 192.168.2.1/24
ifIndex 3 --> 192.168.3.1/24
And the ifTable looks like this:
ifIndex 1 --> Ethernet 10/100
ifIndex 3 --> Ethernet 10/100
Then the interface description for ifIndex 2 will be listed as "[[Not in ifTable]]".