The value to be used as the Authentication Key in Level 2 Hello Packets
whenever the value of usdIsisSysL2CircAuthenticationKeyType has a value
of hmacMd5. A modification of usdIsisSysL2CircAuthenticationKeyType does
not modify the usdIsisSysL2CircAuthenticationPwd value.
Reading this object always results in an OCTET STRING of length zero;
authentication may not be bypassed by reading the MIB object.