3.2 Configuration Model

Why This Topic Matters

The Junos commit model is what separates Junos from many other network operating systems, and Configuration Basics is the single largest JNCIA-Junos objective area (about one-fifth of the exam). Candidates lose points by assuming a set command takes effect immediately - it does not. Expect scenario questions that hinge on the difference between candidate and active configuration, and on which recovery command to use.

Candidate vs. Active Configuration

Junos maintains two distinct configurations:

When you enter configuration mode, Junos copies the active configuration into the candidate. You modify the candidate, review it, then commit to copy it back into the active role and apply it. If you log out without committing, the candidate edits remain pending for shared-config users or are discarded in private mode - so the rule to remember is: nothing happens until you commit.

The Configuration Hierarchy

Junos configuration is a structured tree, not a flat list of lines. The top of the tree is [edit]. Beneath it are stanzas such as system, interfaces, protocols, routing-options, and policy-options, each of which contains sub-levels.

You can display the configuration in two formats. The default hierarchical (curly-brace) format shows the tree; the set format shows the equivalent flat commands.

[edit]
user@host# show system
host-name router1;
services {
    ssh;
}

[edit]
user@host# show system | display set
set system host-name router1
set system services ssh

Core Editing Commands

Using edit changes the [edit ...] banner so you can issue shorter relative commands:

[edit]
user@host# edit interfaces ge-0/0/0 unit 0

[edit interfaces ge-0/0/0 unit 0]
user@host# set family inet address 192.0.2.1/30

[edit interfaces ge-0/0/0 unit 0]
user@host# top

Reviewing Changes Before Commit

Before committing, compare the candidate against the active configuration. This is one of the most exam-relevant habits.

[edit]
user@host# show | compare
[edit system]
+   host-name router1;
[edit interfaces ge-0/0/0 unit 0 family inet]
-   address 10.0.0.1/30;
+   address 192.0.2.1/30;

Lines beginning with + are additions in the candidate; lines beginning with - are removals relative to the active configuration. This shows exactly what a commit would change.

Commit and Its Variants

The commit command validates the candidate, and if it passes, copies it into the active role and applies it. Junos checks syntax and many semantic constraints during commit; a failed check leaves the active configuration untouched.

commit confirmed is critical for remote work: if a change cuts off your own management connection, the device automatically reverts after the timer, so you can reconnect. To keep the change permanently, run a plain commit before the timer expires.

[edit]
user@host# commit confirmed 5
commit confirmed will be automatically rolled back in 5 minutes unless confirmed
commit complete

[edit]
user@host# commit
commit complete

Rollback, Rescue, and Factory Default

Junos automatically archives previous configurations and provides several recovery paths. Knowing which one to choose is heavily tested.

Configuration Rollback

Junos stores the current active configuration plus up to 49 previous versions (so rollback 0 through rollback 49, 50 total). Rollback loads an archived configuration into the candidate - you still must commit for it to become active.

A common exam point: rollback 0 is the standard way to abandon edits you do not want, and after any rollback you must commit to apply it.

Rescue Configuration

A rescue configuration is a known-good configuration that an administrator deliberately saves as a safety net. It is not automatic.

user@host> request system configuration rescue save

[edit]
user@host# rollback rescue
load complete
user@host# commit

Use request system configuration rescue save (operational mode) to capture the current active config as the rescue copy, and rollback rescue (configuration mode) to restore it. request system configuration rescue delete removes it.

Factory-Default Configuration

load factory-default replaces the candidate with the minimal configuration the device shipped with. Because the factory default has no root authentication, Junos requires you to set a root password before you can commit.

[edit]
user@host# load factory-default
user@host# set system root-authentication plain-text-password
New password:
user@host# commit