The move follows Lucent's initial suit, filed in June.
At issue in the suits by both companies are a series of technology patents. Cisco alleges in a suit filed in U.S. District Court in Delaware that Lucent infringed upon six of its patents, and in a separate action filed in the Northern District of California that it infringed upon five other patents.
The Cisco patents in question cover local area switching and asynchronous transfer mode (ATM) technologies.
Cisco also denied liability for patent infringement in response to Lucent's suit.
"This is very commonplace in the industry to countersue," noted a Lucent spokesman.
On Cisco's most recent earnings conference call, president and CEO John Chambers said Lucent's initial action was intended to slow the pace of innovation at Cisco. Industry observers noted that such lawsuits often are used to distract competitors in the market.
In a statement, Kevin Kennedy, senior vice president of Cisco's service provider line of business, painted Lucent as an industry bully: "This is not a case of Cisco vs. Lucent, but rather Lucent vs. the industry. Lucent has sued or threatened to sue the majority of data networking players, large and small."
A Lucent spokesman responded: "This is...not true. Lucent has, in fact, sued a total of two networking companies," including Cisco.
In related news, Cisco today appointed four new senior vice presidents, including Kennedy, who now is assuming control over the company's service provider line of business. Other appointments include Charlie Giancarlo in global alliances, Barbara Beck in human resources, and Cliff Meltzer in IOS technology and engineering operations.