Cisco switches hit by global reboot loop triggered by DNS client failure

Multiple models of Cisco network switches began entering continuous reboot loops early Tuesday after encountering fatal errors in their internal DNS client service, disrupting enterprise and small business networks worldwide.

The failures started at approximately 2 a.m. when affected switches began treating DNS lookup failures as fatal conditions. Devices repeatedly rebooted within minutes of startup, preventing stable network operations. Logs generated immediately before each reboot show fatal DNS client errors indicating an inability to resolve the domain www.cisco.com and network time protocol servers.

The issue impacts a broad range of switches manufactured by Cisco Systems, a U.S.-based networking hardware and software provider. Confirmed affected models include the Cisco CBS250 series, CBS350 series including the CBS350-24P-4G, Catalyst C1200 series, SG350, SG350X, and SG550X series.

Administrators observed that the reboot cycle repeats every few minutes, effectively rendering the switches unusable in production environments. Reports from separate organizations indicate the failures began at roughly the same time across independent networks, pointing to a globally triggered event or a time-based firmware condition rather than localized configuration issues.

Analysis of system logs shows the fatal errors originate from the DNSC task, the switches’ internal DNS client process. The errors occur during attempts to resolve external domains and synchronize time, even in environments where DNS servers are reachable and functioning normally.

Cisco support has acknowledged the issue to customers and confirmed that it affects CBS, SG, and Catalyst 1200 and 1300 series switches. The company has not publicly disclosed the root cause or issued a permanent fix.

In the absence of an official patch, network administrators have implemented temporary mitigation steps to halt the reboot loops. These include disabling DNS resolution on the switches, turning off SNTP or time synchronization, or blocking outbound internet access from switch management interfaces. Disabling DNS settings has been widely reported to immediately stop the reboot behavior.

Cisco has not released further guidance or a formal advisory as of publication. Network operators continue to rely on workarounds while awaiting an official resolution.