The value of tmnxLocUsrDbDhcpIpv6PfxLen indicates
the length of the prefix-address stored in tmnxLocUsrDbDhcpIpv6Pfx.
A value of 0 means that tmnxLocUsrDbDhcpIpv6Pfx is not configured.
The value of tmnxLocUsrDbDhcpIpv6PfxLen must be set together with
the value of tmnxLocUsrDbDhcpIpv6Pfx.