Возможности интерпретатора Python в NX-OS

Возможности интерпретатора Python в NX-OS
Anton Tugai
Customer Support Engineer, Cisco TAC
October, 2014

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© 2013-2014 Cisco and/or its affiliates. All rights reserved.
Cisco Support Community – Expert Series Webcast
Сегодня на семинаре Эксперт Cisco TAC Антон Тугай расскажет о тенденциях в области Cisco SDN и существующих решениях на данный момент.
Антон Тугай
Инженер центра
технической поддержки
Cisco TAC в Брюсселе

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Технические Эксперты
Тема: Возможности интерпретатора Python в NX-OS
Дата проведения вебинара: 21 октября 2014 года
Борис Берлог
Инженер центра технической поддержки Cisco TAC в Брюсселе
Александр Нестеров

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Спасибо, что посетили наш вебинар сегодня
Сегодняшняя презентация включает опросы аудитории
Пожалуйста, участвуйте!

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Вопрос №1
Сталкивались ли вы уже с Openflow / SDN / onePK
a)Нет, не слышал ничего
b)Слышал о существовании но не вникал о чем это
c)Слышал, интересовался, имею представление
d)Уже использовал или ближайшее время планируется запуск, внедрение
e)Куда я попал?

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Cisco Support Community – Expert Series Webcast
Антон Тугай
Октябрь, 2014
Возможности интерпретатора Python в NX-OS

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Несколько слов о Python
Интеграция Python в NX-OS
Примеры и демонстрация
Содержание

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Несколько слов о Python

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Where does the name python come from ?
Trivia!

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Python is an interpreted programming (scripting) language
Available for Linux, Windows, Mac
Two common versions: 2.7(x) and 3.3(x)
2.7 more widely used than 3.3 at the time of writing
Python code is not compiled into standalone binary
It’s either interpreted (interactive shell) or translated into byte code and executed by Python VM
Code can be packaged as a “frozen binary” and executed on systems that don’t have Python installed
the Python VM is packaged within the frozen binary (using py2exe, pyinstaller or freeze)
Basics

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Python home page
http://www.python.org/
Major Releases
Python 3.2.2 September 4, 2011
Python 3.1.4 June 11, 2011
Python 3.0.1 February 13, 2009
Python 2.7.2 June 11, 2011
Python 2.6.7 June 3, 2011
Python 2.5.4 December 23, 2008
Python 2.4.6 December 19, 2008
Python 2.3.7 March 11, 2008
Python 2.2.3 May 30, 2003
Python 2.1.3 April 8, 2002
Python 2.0.1 June 2001
Python 1.6.1 September 2000
Python Distributions
2.7.x is probably the version most frequently used today; this slide deck focuses primarily on 2.7

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Python is an interpreted language
no pre-compile step as in Java and C++
each time Python code is run it is interpreted afresh
code changes can be very quickly made and tested
code can also be entered interactively
can dynamically construct Python code
For example in the form of a string and execute it directly
What about performance?
(much) slower than compiled C/C++ .. but also much more flexible
Good enough for most cases
Very large projects such as Openstack are implemented in Python
Modules to measure (profile) performance exist
Python is interpreted

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When executing Python instructions, Python internally compiles the source into a format called byte code
This is meant to speed up execution
This process is hidden from the programmer
You will now and then notice .pyc files automatically created in your folders (depending if Python has write access)
Next time you run your code, assuming it hasn’t been modified, Python skips the compilation step and directly runs the byte code inside the Python VM
The Python VM is Python’s runtime engine. It always comes with any installation of Python
Note: VM does not mean a VMware VM here!
Byte code compilation

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If you are used to other programming languages that are compiled, you should know there is a significant difference in terms of error checking with interpreted languages
Python checks errors at runtime
This means your code can contain errors such as
if name == ‘Foobar': print repeeeet(name) else: print repeat(name)
as long as name is not Foobar the error won’t get caught!
Keep that in mind, you will get caught!
Interpreted code: error checking

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Be aware that Python is case sensitive
A and a are different variables
We’re going to explore variables in a bit
Python programs typically have the .py extension
Run using python <code.py> or python –i <code.py>
In Linux, you can set the execute bit and the proper shebang
typically, let the shell figure out which Python to use with this shebang: #!/usr/bin/env python
Python is case-sensitive

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You MUST indent code blocks in Python
Keep this rule in mind, it will catch you almost inevitably if you are already used to other programming languages
There are no curly braces to delimitate code blocks
Code blocks are identified with a colon and indentation
Prefer white spaces to tabs for indentation
Four white spaces is the preferred de-facto standard
http://legacy.python.org/dev/peps/pep-0008/#indentation
Indentation is mandatory

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The hash # sign indicates comments follow
Usually ok for short comments
Can start a new line or be used at the end of a line
Comments included at the beginning of your code appear when invoking help(yourscript):
Comments

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Your code should contain comments
They should be relatively short and to the point
Comments should say why you are doing what you are doing
The code already says how; why you did things in a certain way is more important
Too many comments hurt readability though and require maintenance – find the right balance
Sidenote: should you write comments?

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Why Python?
• CLI has no language structures (loops, conditions), Python provides that.
• Established, Modern and Powerful, Clean, lots of libraries.
 TCL: less well-known, archaic, not cool.
 Perl: well-known, but hacky, not object-oriented.
• Applications
Embedded Event Manager (EEM)
Power-On Auto-Provisioning (POAP)
Create “Super Commands”
Quick, Interactive, Automation.

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Python Integration Overview
•Python Version 2.7.2
 Interactive Mode
 Non-interactive Mode (run scripts)
•Enhanced python: new function: cli(“cli-command”)
For show commands, 3 formats:
oReturn classical cli output (raw string)
oReturn a python dictionary (associative array)
oDirect print to stdout (practical when “playing” interactively)
•Python Exception Handling
Syntax Errors
Backend Errors
•Sandbox (no linux shell access, except allowed cases)

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Интеграция Python в NX-OS

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Native python interpreter on nexus
Ability to run python scripts on the nexus switch.
Provides ability to execute CLI commands from your python script.
Python scripts can parse CLI outputs and perform conditional actions (e.g syslog, shut/no shut, logging etc.)
Integration with EEM.
Call a different python script from a script
Lots of python modules available to import in your code.
No license needed!
No license needed
Native Python Available from: Nexus 5000 – 5.2(1)N1(1) Nexus 6000 – day 1 Nexus 7000 – 6.1(2) Nexus 9000 – day 1
Python version :
Nexus 5000 – 2.7.2
Nexus 6000 – 2.7.2
Nexus 7000 – 2.7.2
Nexus 9000 – 2.7.5

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Invoking Python on Nexus 7K-5K
Nexus supports Python v2.7.2 version in 2 modes
ointeractive mode
o non interactive (script) mode
Interactive Mode
switch# python
Copyright (c) 2001-2012 Python Software Foundation; All Rights Reserved
switch# >>> print "hello world“
hello world
switch# >>> exit()
Non Interactive (script) Mode
Switch # source crc.py
----------------------------------------- -------
Started running CRC checker script
finished running CRC checker script
----------------------------------------- --------
Switch # dir bootflash:scripts
946 Oct 30 14:50:36 2013 crc.py
7009 Sep 19 10:38:39 2013 myScript.py
Type python to enter interactive python interpreter
How to call a python script on nexus 7000. crc.py script will be covered later in the slides
bootflash:scripts directory is the default script directory.

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Available on N3K, N5K, N6K, N7K in “recent” releases
Available on N9K “standalone” at FCS
Just type “python” at the prompt
Then import cisco and import cli (N9K)
Python interpreter on NxOS

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Unfortunately at the time of writing, Python on NXOS isn’t exactly similar across all Nexus platforms
The N9K is leading the effort – modules were simplified and cleaned up, XML output is being flattened for several commands
A significant difference between the N9K and the other platforms is how you invoke CLI commands. On the N9K you import cli and you get three functions: cli, clid and clip
cli.cli returns a string
cli.clid returns a JSON string
cli.clip returns print-friendly CLI output
Not platform independent

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How is Python integrated? (1/2)
CLI Interpreter
Python Interpreter
Operator
Console/telnet/ssh
Other nxos component
(bgp, osfp, …)
MTS
VSH
Switch from CLI to Python Interpreter
o Interactive: python / exit
o Non-interactive: source <file>
However, In Python Interpreter,
? and <tab> (online help)
operate on CLI parse-tree
py
exit
Cli()

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How is Python integrated? (2/2)
CLI Interpreter
Python Interpreter
Operator
Console/telnet/ssh
Other nxos component
(bgp, osfp, …)
MTS
VSH
•All regular python commands
o May be crippled by sandbox
o to block potentially malicious calls
• New python command “cli()”
o Run cli commands in python
• e.g, cli("show version")
o Takes cli command arg as a string
o Executes the cli command through
• cli interpreter
o Returns results as a string or
• dictionnary
o
Filtered by sandbox
Results as a string
CLI commands as a string
Execution

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switch# show clock
23:54:55.872 UTC Wed May 16 2012
switch# python -- enter python interpreter (adding >>> or ... to prompt)
switch# >>> cli(“conf term ; interface loopback 1”)
switch(config-if)# >>> cli(“ip address 1.1.1.1/24”)
switch(config-if)# >>> cli(“exit”) -- exit the cli interface mode
switch(config)# >>> cli(“exit”) -- still in python interp mode
switch# >>> i=0
switch# >>> while i<8: -- composite command -> ‘more input’ prompt (...)
switch# ... i=i+1
switch# ... cmd = "show module %i" % i
switch# ... r=clid(cmd)
switch# ... if "TABLE_modinfo/model" in r.keys():
switch# ... if r["TABLE_modinfo/model"] == "Nurburgring":
switch# ... print "got a racer in slot %d" % i
switch# ... – empty input to indicate end of loop
got a racer in slot 3
switch# >>> exit -- return to cli interpreter
switch#

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switch# python -- create python interpreter
switch# >>> i = 2
switch# >>> print “var i = %d” % i
var i = 2
switch# >>> cli(“configure terminal”)
switch(config)# >>> blabla
switch(config)# >>> exit -- destroy python interpreter
switch# - cli interp still at exec mode (“conf t” is lost)
switch# python -- new python interp
switch# >>> print “var i = %d” % i -- previous python cli mode and vars gone
Error: variable ‘i’ undefined.
switch# >>> exit
switch# conf t ; inter lo 1
switch(config-if)# python -- new python interp
switch(config-if)# >>> -- inherits the cli mode (forked from cli).
Python is forked from vsh ->
•No state is preserved between two python invocations
•CLI mode is lost permanently when exiting python

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CLI Formats (1/2)
string = cli (“cli-command”) -- returns cli output as a string
dictionary = clid (“cli-command”) -- returns a dictionary (if xml output)
clip (“cli-command”) -- returns void, prints to stdout
Three formats:
switch# >>> cli("conf ; interface loopback 1")
Enter configuration commands, one per line. End with CNTL/Z.
switch(config-if)# >>> clip('where detail‘)
mode: conf
interface loopback1
username: root
vdc: switch
routing-context vrf: default
switch(config-if)# >>> cli('where detail')
'x1b[00m mode: confn interface loopback1n username: rootn vdc: switchn routing-context vrf: defaultn'
switch(config-if)# >>> r = cli('where detail') ; print r
(same output as clip() above!)

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switch(config-if)# >>> i=0
switch(config-if)# >>> while i<3:
switch(config-if)# ... i=i+1
switch(config-if)# ... cli('ip addr 1.1.1.1/24')
switch(config-if)# ...
switch(config-if)# >>> cli('end')
switch# >>> r = clid('show version')
switch# >>> for k in r.keys():
switch# ... print "%30s" % k, " = %s" % r[k]
switch# ...
cpu_name = Intel(R) Xeon(R) CPU
rr_sys_ver = 6.2(0.110)
manufacturer = Cisco Systems, Inc.
isan_file_name = bootflash:///full
rr_ctime = Wed May 16 02:40:57 2012
proc_board_id = JAF1417AGCB
bios_cmpl_time = 02/20/10
kickstart_ver_str = 6.1(1) [build 6.1(0.292)] [gdb]
isan_tmstmp = 05/16/2012 02:26:02
switch# >>> exit

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switch# show file bootflash:scripts/test1.py
#!/bin/env python
i=0
while i<3:
r=clip('show version')
uptime_name='/@/show/version/__readonly__/kern_uptm_secs'
print uptime_name, r[uptime_name]
clid('sleep 1')
i=i+1
switch# source test1.py -- default directory is /bootflash/scripts
/@/show/version/__readonly__/kern_uptm_secs 36
switch#
Default directory of scripts
Invoke python

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switch(config)# >>> cli("interface loop 1")
switch(config-if)# >>> cli("ip address 1234.1.1.1/24")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
cisco.cli_syntax_error: % Invalid ip address at '===>' marker:
% ip address 123===>4.1.1.1/24
switch(config-if)# >>> try: cli("ip address 1234.1.1.1/24")
switch(config-if)# ... except: print sys.exc_type, sys.exc_value
<class 'cisco.cli_syntax_error'> % Invalid ip address at '===>' marker:
% ip address 123===>4.1.1.1/24
switch(config-if)# >>> cli("interface loopback 2")
<class 'cisco.cli_execution_error'>
% 1.1.1.1/24 overlaps with address configured on loopback1
switch(config-if)# >>>

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switch# python
switch# >>> import os
switch# >>> os.getcwd()
'/bootflash'
switch# >>> os.chdir("/isan/bin")
Permission denied. Traceback (most recent call last):
OSError: [Errno 13] Permission denied: '/isan/bin'
switch# >>> os.system("cd /isan/bin")
system(cd /isan/bin): rejected!
-1
switch# >>> f=open("/isan/bin/vsh", "r")
Permission denied. Traceback (most recent call last):
IOError: [Errno 13] Permission denied: '/isan/bin/vsh'
switch# >>> f=open("/bootflash/alias", "r")
switch# >>>
•Block potentially harmful function calls

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An “online help protocol” allows script to provide online help, man-page and context sensitive variants
switch# source ? abc No help  script is not supporting the online help protocol cgrep 'context grep': shows matching line plus the context lines (to be used behind a pipe, for show-run type of output) Example of context for 'ip address 1.1.1.1/24': ‘eth 2/1’ find-if Find interfaces that match selected attribute values (from 'show intf br') ntimes Runs specified command specified numbers of times redo-history This is run a list of commands from the history again show-if Show selected interface attributes show-version A better 'show version' sys/ Directory here you find example scripts, packaged in image switch# source ntimes ?  filename can be abbridged if unique (or tabbed) arg1: the command, in quotes arg2: number of times to run the commands <CR> > Redirect it to a file >> Redirect it to a file in append mode | Pipe command output to filter

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The script is called with the following argument: __cli_script.*help __cli_script_help : request to return a one line description of the script’s purpose __cli_script_args_help : request to return help for arguments, filename is passed as first argument, as well as any arguments entered so far __cli_script_args_help_partial: the last argument is ‘partial’. E.g. “token?” instead of “token ?”. Argument help return formats: classical (“type|description” left|right side”): print "ftp|Use ftp for file transfer protocol" print "scp|Use scp for file transfer protocol" exit(0) Man Page style (simpler to implement, but no tabbing). print "__man_page" print " whatever…“

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Python Introspection on Cisco Extension >>> import cisco >>> cisco.__doc__ 'contains commands that integrate with Cisco CLI' >>> dir(cisco) ['__doc__', '__name__', '__package__', 'cli', 'clid', 'clip'] >>> cisco.cli.__doc__ 'execute a cli command, return the lines of output as char*' >>> cisco.clid.__doc__ 'execute a cli command, return name/value pairs (d: dictionary)' >>> cisco.clip.__doc__ 'execute a cli command, just dump to stdout, return void to python (p: print)' >>> >>> [(a,type(cisco.__getattribute__(a))) for a in dir(cisco)] [('__doc__', <type 'str'>), ('__name__', <type 'str'>), ('__package__', <type 'NoneType'>), ('cli', <type 'builtin_function_or_method'>), ('clid', <type 'builtin_function_or_method'>), ('clip', <type 'builtin_function_or_method'>)] >>>

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Backgrounding a script
switch# source background sleep 100
switch# source background sleep 50
switch# show background
username . terminal pid start time script args ...
root . . pts/0 7687 03:31 00:00:00 sleep.py 90 .
root . . pts/0 7696 03:31 00:00:00 sleep.py 50 .
switch# kill background ?
WORD Background script to terminate, by process-id or just
a regex matching any line from 'show background' output
switch# kill background 7687
switch# show background
username . terminal pid start time script args ...
root . . pts/0 7696 03:31 00:00:00 sleep.py 50 .
switch#
switch# exit
Linux(debug)# su john
switch# kill background 7696
bash: line 1: kill: (7696) - Operation not permitted
switch#

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Expect
switch# python
switch# >>> import pexpect
switch# >>> child=pexpect.spawn("vsh") -- only vsh allowed
switch# >>> child.sendline("show clock")
11
switch# >>> child.expect(".*2012")
0
switch# >>> print child.after
show clock
switch# show clock
03:44:12.620 UTC Wed Sep 26 2012
switch# >>>

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Importing Python Libraries Containing cli()
You need to import the ‘cisco’ module
(done automatically by ‘python’ or ‘source’ command, but not by ‘import’ command).
File time.py:
from cisco import cli
def show_clock:
cli(“show clock”)
File stuff.py:
import time
time.show_clock()

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Вопрос №2
После полученной информации ваш интерес к SDN
a)У меня и так все работает, не хочу усложнять и строить космический корабль
b)Интересно, возможно протестирую в свободное время
c)Заинтересован, займусь изучением вопроса и попробую запустить
d)У нас уже используется либо проводятся тесты
e)До сих пор не понимаю о чем идет речь

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Вопрос №3
О чем было бы интересно узнать на следующем вебинаре о SDN
a)Контроллерные решения на основе Cisco XNC
b)Cisco и OpenStack совместные решения
c)APIC / ACI / Insieme / Nexus 9000
d)VxLAN

Отправьте свой вопрос сейчас!
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Вебинар на русском языке Тема: Базовая настройка Device Provisioning и отладка основных проблем при использовании Cisco TMS Provisioning Extension
во вторник, 25 ноября, в 12.00 мск
Присоединяйтесь к эксперту Cisco
Михаилу Щекотилову
В рамках сессии будет проведена демонстрация базовой настройки Device Provisioning на Cisco VCS и TMS, а также разобраны основные ошибки, которые при этом встречаются, и показаны методы и инструменты для их отладки.

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•Русском  https://supportforums.cisco.com/community/russian
•Испанском  https://supportforums.cisco.com/community/5591/comunidad-de- soporte-de-cisco-en-espanol
•Португальском  https://supportforums.cisco.com/community/5141/comunidade-de-suporte- cisco-em-portugues
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