Cisco IOS – Programming, Automation, and Information

1. Almost every^{Note 1} Cisco IOS system running at least IOS 12.2 has built-in tcl shell programmability. You can store tcl scripts in flash and run them.

2. Cisco IOS also has a light native programming environment called EEM (Embedded Event Manager). EEM events can be triggered by a wide variety of inputs, such as packets on a certain port (via Netflow), log messages, or interface up/down. Think of EEM as a subset of tclsh capabilities in IOS; EEM sucks less than programming in TCL, but you also get spotty feature support depending on the IOS mix you are dealing with. EEM can be run on a cron schedule if that suits your needs. At the bottom of this answer, I included an example EEM script; keep in mind that EEM is best for small-ish numbers of "if-this, do-that" statements... when you want to start defining functions and such, just bite the bullet and use tclsh.

3. New Cisco products support a special set of canned SDN APIs which are broader than I can explain while typing on my mobile phone; one example from this family includes Cisco APIC, but that's just scratching the surface of what they have available.

4. I would be remiss if I didn't point out that most of the Cisco scripting done in the last 20 years has been via screen-scraping, snmp, and (these-days) NETCONF. Anything you can do from the CLI, can be automated via screen-scraping; I have spent a large portion of my career doing just this. Screen-scrapers usually host their scripts on an external linux system and connect to a router with the weapon of their own choice... usually that's one of these languages:

   • TCL / Expect
   • Perl... see Net::SSH::Perl, Net::Telnet::Cisco, Net::SNMP, and Cisco::Reconfig
   • Python... see pexpect, netmiko, exscript, pysnmp, and ciscoconfparse
   • Ruby... see net-ssh, snmp, and cisco

5. Finally, the newer Cisco Nexus products have a Python API (such as this one for the Nexus 9K); python APIs are typically easier to deal with than TCL APIs, but at this point you're restricted to the Nexus family.

^{Note 1} The only exception I know of is the Cisco 3550, which doesn't have tclsh in 12.2.