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