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Introduction to Juniper Junos

April 23, 2012
Brad Wilson

None of us have much time to waste, so what can you do when your environment changes quickly and you need to come up to speed on a device that you have never seen before? You find the quickest way to bring yourself up to speed. Some of you will have experience with other vendors’ equipment, and some of you will not have much experience at all; therefore, we will focus on topics that will get your feet wet the quickest and have you talking Junos in no time.

The Junos OS

The core of Juniper devices is the Junos OS. Junos OS is based on the FreeBSD UNIX operating system (OS). The OS implements a single software train through the entire line of Junos products. The same train is used from the entry-level J series routers to the service provider TX Matrix switches.

To create stable devices that can deliver high performance you need to adhere to sound logic and strict processes. Juniper has done this by keeping a clean separation between the control and forwarding planes on their devices. The control plane includes the routing engine and forwarding table. The forwarding table is linked to the packet-forwarding engine of the forwarding plane, allowing for stable control protocols and high speed forwarding capabilities. The forwarding plane is responsible for the other functions like policing, stateless firewall filtering, and class of service. Transit traffic is forwarded via the Packet Forwarding Engine without passing through the control plane. This is possible because the control plane updates the forwarding table for the forwarding plane. The exception traffic, traffic destined to the local device, is sent to the routing engine CPU.

Configuring Junos Devices


Junos has the capability for a user to automatically login to the UNIX BSD shell just like you would normally with a Linux or UNIX device. For example, if you login as root, you will be placed into the UNIX BSD shell. You can’t perform any switching, routing, or security functions from the BSD shell. You must switch to operation mode where you can display statistics and configuration information. The UNIX BSD shell is designated with the % prompt. Here you can enter standard UNIX commands.


Operational Mode

You can enter operation mode by typing cli and pressing enter. Operational mode is designated with the > prompt. Once you’re in operation mode, you can utilize operational commands, and you can enter configuration mode from here.

cli root>

Configuration Mode

Entering configure or edit at the operational prompt will place you in configuration mode. Configuration mode is designated with the # prompt. You will also notice that the word edit appears above the prompt in brackets. This shows your level in the command hierarchy and it will change as you change levels.

root> configure
Entering configuration mode

Once you enter configuration mode, you are placed at the at the most global and least specific section of the configuration. You can now move directly to the section of the configuration that you would like to configure. For example, if you typed edit interfaces, you would move to the interface level. Once you’re at the interface level and you display the configuration, you will only see interface-level information. This is a nice feature when you have a large configuration, and you only wish to configure a specific part of the configuration.

You do have the option to display the configuration in Junos the same way that you displayed it in Netscreen Screen OS. This is accomplished with the show configuration command with the pipe option and the display set option.

show configuration | display set

Saving Configurations

You use the commit command to save a configuration to the Juniper device. If you make a mistake with a configuration and you save the configuration, you can use the rollback command to use a previous configuration. Actually, you can choose to use one of the last 50 saved configurations.

Monitoring, Maintaining and Troubleshooting Junos Devices


You have many commands in operational mode to display statistics about the Junos device. And, as with most vendors, you can send SNMP traps and SYSLOG messages to a server. Since the Junos device is a UNIX device you will also have access to all of the standard UNIX log files, and the log files are still available after a reboot since they are not stored in RAM.


One thing to keep in mind is that the Junos devices are not like a traditional router since they are based on a UNIX platform. The devices have directory and file structures just like a UNIX server, and it should be treated with care. You should follow the proper procedures when you want to shutdown a Junos device.


For troubleshooting you have a couple of options. Junos has the traceoptions function which is the equivalent of other vendors debug function. The traceoptions function is controlled by the traceoptions command. If someone is performing a trace, they are performing the same function as a debug. You can trace many protocols on the Junos devices. The traceoptions function will create a file on the local device. You can view the trace file with the show log command. There is also the monitor function. The monitor function will allow you to monitor in the log files on the Junos device in real-time. It is the equivalent of the tail -f command on UNIX devices. The monitor command used to control the monitor function. The Junos device writes many log files. This is much different than most vendors.

There are many exciting features in the Junos OS. As I mentioned you have direct access to UNIX shell. This comes in handy when you’re troubleshooting with the Junos devices. And you should feel at ease to know that there is a single OS train for all of the Juniper products. Also, take advantage of the help command. You will come up to speed quicker since it’s like having the manual at your finger tips.

Related Course
Junos Foundations: JNCIA-Junos Boot Camp (IJOS, JRE)

Excerpted from Global Knowledge White Paper: Introduction to Junos