Someone asked me the other day how they could automate the execution of a command on multiple routers without accessing each router manually. Obviously an Ansible playbook can easily do that (or even using Ansible ad-hoc command without a playbook); or you can write a bash script with a for loop that iterates over the devices and connects to each device to run the command. But that question also got me wondering if there was a simpler way to get the job done quickly without installing any configuration management tools or messing with scripting?

It turns out the answer is Yes. You can do that using good old SSH — specifically using parallel SSH on Linux.   

Parallel SSH (PSSH) is a great tool to use when you want to run single or multiple commands on more than one host or router at the same time. All what you need is a Linux host with PSSH installed and you are good to go. You can install PSSH on Ubuntu by using the Python package installer pip install pssh (if you don’t have the Python package installed, you can install it by executing apt-get install python-pip).

pssh [OPTIONS] command […]

Let’s look at a quick example. I have two routers, csr and csr3, defined in the hosts.txt file and I want PSSH to save the runnng configuration on each router. The optional -l argument tells PSSH what unsername to use while the -A argument tells it to prompt for a password (alternatively you can use private/public key pair instead of passwords).

➜  ~ pssh -h hosts.txt -l cisco -A “wr mem”

Warning: do not enter your password if anyone else has superuser privileges or access to your account.

Password:

[1] 20:11:59 [SUCCESS] csr3

[2] 20:12:00 [SUCCESS] csr

 

If you want to gather some data from the routers and write the output to a file, you can do so by adding the -o argument as follows:

➜  ~ pssh -h hosts.txt -l cisco -o/tmp/out/ -A “show run”

Warning: do not enter your password if anyone else has superuser privileges or access to your account.

Password:

[1] 20:14:39 [SUCCESS] csr3

[2] 20:14:39 [SUCCESS] csr

➜  ~

➜  ~ ls /tmp/out/

csr  csr3

➜  ~

➜  ~ more /tmp/out/csr

 

Building configuration…

 

Current configuration : 2643 bytes

!

! Last configuration change at 14:59:27 UTC Wed Feb 8 2017 by cisco

!

version 15.4

service timestamps debug datetime msec

service timestamps log datetime msec

no platform punt-keepalive disable-kernel-core

platform console virtual

!

hostname CSR

!

➜  ~

 

The PSSH utility is lightweight, simple, and does the job with minimum overhead. It also runs through routers in parallel which saves time especially when you are executing tasks that take some time to complete. 

It’s amazing how much you can do with the mighty SSH. Keep PSSH in your toolbox in case you need it one day. 

Did I say there is also PSCP (parallel SCP) utility which you can use to copy an image to multiple devices at the same time when you are upgrading those devices? That is your homework now, Google it and check it out. 


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