Подключаемся консолью к маршрутизатору и включаем его Ждем “Hit [Enter] to boot immediately, or space bar for command prompt.”
и нажимаем пробел. Появится приглашение “loader>”.

Загружаемся в single mode
loader> boot -s
Kernel entry at 0x801000d8 …

Дожидаемся “Enter full pathname of shell or 'recovery' for root password recovery or RETURN for /bin/sh:”
и вводим команду “recovery”.
Ждем приглашения “root>”.

NOTE: Once in the CLI, you will need to enter configuration mode using
NOTE: the 'configure' command to make any required changes. For example,
NOTE: to reset the root password, type:
NOTE:    configure
NOTE:    set system root-authentication plain-text-password
NOTE:    (enter the new password when asked)
NOTE:    commit
NOTE:    exit
NOTE:    exit
NOTE: When you exit the CLI, you will be asked if you want to reboot
NOTE: the system

Starting CLI ... 

root> 

Задаем новый пароль root, применяем конфигурацию и перезагружаемся

root> configure 
Entering configuration mode

[edit]
root# set system root-authentication plain-text-password     
New password:
Retype new password:

[edit]
root# commit 
commit complete

[edit]
root# exit 
Exiting configuration mode

root> request system reboot 
Reboot the system ? [yes,no] (no) yes 
Shutdown NOW!

Подробное описание процедуры:
http://www.juniper.net/documentation/en_US/junos13.3/topics/task/configuration/authentication-root-password-recovering.html