(Updated with Arista's response.)

Arista has been a thorn in Cisco's side, taking away some market share in switching. Now Cisco is claiming that Arista took a lot more — namely, some patentable technology and more than 500 command-line interface (CLI) commands.

Cisco filed a patent-infringement suit and a copyright-infringement suit against Arista today in Federal District Court for the Northern District of California, as General Counsel Mark Chandler explains on Cisco's blog.

Cisco claims Arista's products have incorporated 12 proprietary features covered by 14 patents, 11 of which are listed on the blog posting. The inventions "were patented by individuals who worked for Cisco and are now at Arista, or who at Cisco worked with executives who are now at Arista," Chandler writes.

Among the patents supposedly infringed is No. 7,162,537, for a state-sharing database. Cisco claims this is the "multi-process state-sharing architecture" that Arista CEO Jayshree Ullal, a former Cisco executive, boasted about in 2009 on Arista's blog, where she called it the "secret sauce" of Arista's EOS software.

The copyright-infringement suit covers user manuals (where Cisco claims it found copied grammatical errors) and multiword commands used in Cisco's command-line interface (CLI). Chandler argues in his blog that multiword commands are protected under copyright (whereas obvious single-word commands such as "delete" or "copy" are not). He says more than 500 instances of copied commands were found in Arista's interface, as opposed to 30 in the case of Juniper Networks' Junos.

Cisco claims this is important because Arista has used its familiar, Cisco-like interface as a selling point.

Arista did not immediately respond to a request for comment.

UPDATE 10:00 a.m.: Here's Arista's statement:

"We just became aware of the lawsuit and have not had an opportunity to evaluate the claims in detail. We will certainly be doing so in the coming days. While we have respect for Cisco as a fierce competitor and the dominant player in the market, we are disappointed that they have to resort to litigation rather than simply compete with us in products."

Arista has also provided a personal statement from Ullal:   "I am disappointed at Cisco's tactics. Its not the Cisco I knew."