The value of the object tmnxLocUsrDbPppoePrefixLength
specifies the prefix length of tmnxLocUsrDbPppoeAddress.
The value of the object tmnxLocUsrDbPppoePrefixLength is only
relevant if the value of the object tmnxLocUsrDbPppoeAddrType is not
'unknown';
if the value of the object tmnxLocUsrDbPppoeAddrType is equal to
'unknown', the value of tmnxLocUsrDbPppoePrefixLength is ignored.