Junos monitor interface: Real-Time Traffic Monitoring

When a link starts flapping or a port silently drops errors, monitor interface is the Junos command that shows you what is happening right now — updated every second. Unlike show interfaces, which gives a snapshot, the monitor view tracks delta counters as they change, making it the fastest way to watch a fault develop in real time on EX, QFX and MX platforms.

What monitor interface Does

It displays real-time statistics about interfaces, refreshing every second, and checks for common failures: SONET/SDH and T3 alarms, detected loopbacks, and increases in framing errors. On Junos OS Evolved the command works over SSH sessions, while console and Telnet sessions are not supported.

Starting a Monitoring Session

user@router> monitor interface ge-0/0/1

Watch the Input errors, Framing errors and L2 incompletes columns. On a healthy link these stay at zero; a climbing counter under load indicates a physical-layer problem (bad optics, marginal cable, duplex mismatch) rather than a routing issue.

Live Delta Counters with the Traffic View

The traffic variant shows byte and packet rates with delta counters since the session started, letting you quantify throughput per interface during maintenance windows:

user@switch> monitor interface traffic
Interface    Link  Input packets        (pps)     Output packets        (pps) Description
ge-0/0/1       Up      392172          (0)             392187          (0)
ge-0/0/22      Up      392169          (0)             392184          (1)

Press c to zero the delta counters mid-session (accumulative counters are untouched — clear those with clear interfaces interval), and p to switch interfaces without restarting the monitor.

Correlating with Interface Damping and Hold Times

If monitoring shows the link bouncing, check whether interface damping is masking the flapping behavior — see our guide on Junos hold-time and damping configuration and how to identify a flapped interface with CLI commands. For a systematic physical-layer sweep, follow the Juniper Ethernet interface troubleshooting checklist.

原文链接：https://www.juniper.net/documentation/en_US/junos/topics/reference/command-summary/monitor-interface.html