The list of trunk-enabled VLANs on a Cisco device is retrieved through the CISCO-VTP-MIB.
The list is made up of a string of 4096 bits for each trunk port, and these 4096 bits are split into four 1024-bit long SNMP table columns.
NAV's cisco_vtp_mib.py implementation concatenates these four columns into one bitstring, but stops concatenation on the first empty bitstring (this is taken as an indicator that there are no more enabled VLANs with higher vlan tags).
Tests from the University of Linköping reveals that this isn't always the case. Some Cisco devices populate only columns that have any set bits at all. If all enabled VLANs are above 2048, for example, the first two columns may be empty and no VLANs are detected by NAV.
The tests also indicate that the bitstrings aren't always padded out to a full 1024 bits, so a simple concatenation isn't enough to ensure a correct result.
fix here: https:/