Step 2: Isolate the Causes

Purpose

A particular symptom can be the result of one or more causes. Narrow down the focus of your search to find each individual cause of the unwanted behavior.

Action

To isolate the cause of a particular problem, enter one or all of the following JUNOS CLI operational mode command:

user@host> show < configuration | bgp | interfaces | isis | ospf | route >

Your particular problem may require the use of more than just the commands listed above. See the appropriate command reference for a more exhaustive list of commonly used operational mode commands.

Sample Output

user@R6> show interfaces terse 

Interface               Admin Link Proto Local                 Remote

so-0/0/0                up    up  

so-0/0/0.0              up    up   inet  10.1.56.2/30    

                                   iso  

so-0/0/2                up    up  

so-0/0/2.0              up    up   inet  10.1.26.2/30    

                                   iso  

so-0/0/3                up    up  

so-0/0/3.0              up    up   inet  10.1.36.2/30    

                                   iso 

[...Output truncated...]

The following sample output is from R2:

user@R2> show route 10.0.0.5 

inet.0: 22 destinations, 25 routes (22 active, 0 holddown, 0 hidden)

+ = Active Route, - = Last Active, * = Both

10.0.0.5/32        *[Static/5] 00:16:21

                    > to 10.1.26.2 via so-0/0/2.0

                    [BGP/170] 3d 20:23:35, MED 5, localpref 100

                      AS path: 65001 I

                    > to 10.1.12.1 via so-0/0/0.0

What It Means

The sample output shows that all interfaces on R6 are up. The output from R2 shows that a static route [Static/5] configured on R2 points to R6 (10.1.26.2) and is the preferred route to R5 because of its low preference value. However, the route is looping from R2 to R6, as indicated by the missing reference to R5 (10.1.15.2).