4.1 Monitoring with Operational Commands

Why Operational Monitoring Matters on the Exam

The Operational Monitoring and Maintenance objective is roughly 13.5% of JNCIA-Junos. Many questions do not ask you to recall a command name; they show you sample operational output and ask what it means or which command produced it. Building the habit of reading show output carefully is the highest-leverage skill in this domain.

All monitoring commands run from operational mode (the > prompt). You do not need to enter configuration mode to inspect device state. Junos pipes (| match, | count, | display json) work on almost every operational command, which the exam expects you to recognize.

Interface Status: show interfaces and monitor interface

show interfaces is the core command for verifying connectivity. Two state lines matter most:

• Physical link state: Physical link is Up or Down (cable, optics, or remote side)
• Admin state: Enabled or Administratively down (set by the disable statement under the interface)

An interface can be administratively enabled while its physical link is down, so always read both lines.

user@router> show interfaces ge-0/0/1 terse
Interface               Admin Link Proto    Local                 Remote
ge-0/0/1                up    up
ge-0/0/1.0              up    up   inet     198.51.100.1/30

Use modifiers to control detail level:

monitor interface ge-0/0/1 opens a full-screen, auto-refreshing view of one interface, showing input/output rate and incrementing error counters in real time. monitor interface traffic shows a refreshing summary table for all interfaces. Press q to quit. Use monitor interface when you must watch counters change live; use show interfaces extensive for a static snapshot.

Routing and System State

show route displays the routing table (RIB) maintained by the Routing Engine (RE). This is the protocol/decision view. It is not the same as the forwarding table (FIB) programmed into the Packet Forwarding Engine (PFE); use show route forwarding-table to see what the PFE actually uses to switch packets.

user@router> show route 203.0.113.0/24

inet.0: 14 destinations, 14 routes (14 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

203.0.113.0/24     *[Static/5] 1d 02:14:33
                    > to 198.51.100.2 via ge-0/0/1.0

The [Static/5] field shows the protocol and its route preference (lower is preferred). The * marks the active route installed into the forwarding table.

Useful system status commands:

• show system uptime - time since last reboot and last config commit
• show system users - who is logged in and from where
• show system storage - filesystem usage (watch /var and /config)
• show system processes extensive - per-process CPU and memory
• show system alarms and show chassis alarms - active red/yellow alarms

On-Box Capture: monitor traffic

monitor traffic interface ge-0/0/1 runs a built-in packet capture (tcpdump-based). The exam stresses one critical limitation: it captures only traffic that is destined to or sourced from the Routing Engine (host-bound / exception traffic such as ping to the device, OSPF, SSH). It does not see normal transit traffic that the PFE forwards between interfaces. To inspect transit traffic you use firewall filters with sample/log/port-mirror actions instead.

Common options: no-resolve (skip DNS lookups), size <bytes> (capture length), count <n> (stop after n packets), and matching "<expr>" (tcpdump filter). Always exit with Ctrl+C so the capture stops cleanly.

Reachability Tools: ping and traceroute

Both run from operational mode and support options that the exam tests:

Stop a continuous ping with Ctrl+C. traceroute reports each hop's address and round-trip time; * * * at a hop usually means ICMP TTL-exceeded replies are filtered, not necessarily a broken path.

Hardware and Environmental Health

For physical health checks, JNCIA-Junos expects familiarity with the show chassis family:

• show chassis hardware - inventory: model, serial numbers, FRU (Field-Replaceable Unit) part numbers
• show chassis environment - temperature sensors, fan speed/status, and power status
• show chassis routing-engine - RE CPU load, memory, and temperature
• show chassis fpc - status and memory of Flexible PIC Concentrators (line cards)
• show chassis alarms - active hardware alarms (a yellow alarm is minor; a red alarm is major)

If show chassis environment reports a fan as Failed or a temperature sensor in the danger range, that is a hardware-level alarm condition you would correlate with show chassis alarms output before any maintenance action.