Advanced Junos Troubleshooting: Key Commands and Techniques

In this blog post, we will delve into some advanced Junos troubleshooting techniques and key commands that are essential for anyone preparing for the JNCIS-ENT certification. Junos, the operating system used in Juniper Networks devices, is renowned for its robustness and flexibility. However, like any complex system, it can sometimes present challenges that require a deep understanding of its inner workings to resolve.

Junos CLI and Operational Mode

The Junos CLI (Command Line Interface) is the primary tool for interacting with Junos devices. It has two main modes: Operational mode and Configuration mode. Operational mode is used for monitoring and troubleshooting, while Configuration mode is used for making configuration changes.

To enter the Operational mode, simply log in to the device. The prompt will look like this:


In Operational mode, you can execute various show and monitor commands to inspect the device's status and performance. For example, the show interfaces command displays the status of all interfaces on the device:

user@device> show interfaces

Key Troubleshooting Commands

Here are some key commands that are particularly useful for troubleshooting:

Packet Capturing with tcpdump

Junos also includes a built-in packet capturing tool called tcpdump. This tool can capture and analyze network traffic directly on the device, which can be invaluable for troubleshooting network problems.

To use tcpdump, you first need to start a shell session with the start shell command:

user@device> start shell

Then, you can use the tcpdump command to start capturing packets. For example, the following command captures packets on the ge-0/0/0 interface:

% sudo tcpdump -i ge-0/0/0

You can also use various options to filter the captured packets and control the output. For example, the following command captures only ICMP packets and displays them in verbose mode:

% sudo tcpdump -i ge-0/0/0 -v icmp


These are just a few of the many tools and techniques available for troubleshooting Junos devices. By mastering these commands and understanding how to interpret their output, you can quickly diagnose and resolve a wide range of network problems. This knowledge is not only essential for the JNCIS-ENT certification but also invaluable for any network engineer working with Junos devices.

© Ben Jacobson.RSS