Step 1: Identify the Symptoms

Purpose

The symptoms of a problem in your network are usually quite obvious, such as the failure to reach a remote host.

Action

To identify the symptoms of a problem on your network, start at one end of your network and follow the routes to the other end, entering all or one of the following JUNOS command-line interfaces (CLI) operational mode commands:

user@host> ping (ip-address | host-name)

user@host> show route (ip-address | host-name)

user@host> traceroute (ip-address | host-name)

Sample Output

user@R6> ping 10.0.0.5 

PING 10.0.0.5 (10.0.0.5): 56 data bytes

36 bytes from 10.1.26.1: Time to live exceeded

Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst

 4  5  00 0054 e2db   0 0000  01  01 a8c6 10.1.26.2  10.0.0.5 

36 bytes from 10.1.26.1: Time to live exceeded

Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst

 4  5  00 0054 e2de   0 0000  01  01 a8c3 10.1.26.2  10.0.0.5 

36 bytes from 10.1.26.1: Time to live exceeded

Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst

 4  5  00 0054 e2e2   0 0000  01  01 a8bf 10.1.26.2  10.0.0.5 

^C

--- 10.0.0.5 ping statistics ---

3 packets transmitted, 0 packets received, 100% packet loss

user@R6> show route 10.0.0.5 

inet.0: 20 destinations, 20 routes (20 active, 0 holddown, 0 hidden)

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

10.0.0.5/32        *[IS-IS/165] 00:02:39, metric 10

                    > to 10.1.26.1 via so-0/0/2.0

user@R6> traceroute 10.0.0.5 

traceroute to 10.0.0.5 (10.0.0.5), 30 hops max, 40 byte packets

 1  10.1.26.1 (10.1.26.1)  0.649 ms  0.521 ms  0.490 ms

 2  10.1.26.2 (10.1.26.2)  0.521 ms  0.537 ms  0.507 ms

 3  10.1.26.1 (10.1.26.1)  0.523 ms  0.536 ms  0.514 ms

 4  10.1.26.2 (10.1.26.2)  0.528 ms  0.551 ms  0.523 ms

 5  10.1.26.1 (10.1.26.1)  0.531 ms  0.550 ms  0.524 ms

What It Means

The sample output shows an unsuccessful ping command in which the packets are being rejected because the time to live is exceeded. The output for the show route command shows the interface (10.1.26.1) that you can examine further for possible problems. The traceroute command shows the loop between 10.1.26.1 (R2) and 10.1.26.2 (R6), as indicated by the continuous repetition of the two interface addresses.