Users of Cisco's ASA firewall and Firepower Threat Defence devices are being warned to urgently reboot the devices or they'll stop working.
A "functional software defect" means the appliances will stop passing traffic after running for 213 days, the company said.
By using the Cisco iOS show version | grep up command, customers can determine the device uptime; the show asp drop command will indicate if the bug has been triggered and packets are being dropped, if the message "punt rate limit exceeded" is displayed.
Cisco said it would publish updates to address the problem in the next few weeks.
In the meantime, it advised customers to work around the issue by rebooting their appliances so the uptime counter resets.
Cisco said the bug was not a vulnerability and cannot be triggered by threat actors.
The company discovered the bug a day ago.
Update 3 April 2017: Cisco has now issued a public field notice that lists which products are affected, and which should be rebooted to continue to work.