Several Cisco switch models are unexpectedly entering reboot loops after reporting critical DNS client errors, according to recent reports compiled by BleepingComputer.
The issue appears to have started around 2 AM, when a firmware problem in the switches’ DNS client service began treating DNS lookup failures as fatal errors. This caused the affected switches to continuously reboot.
Affected devices log critical errors similar to the following before restarting:
DNS_CLIENT - SRCADDRFAIL - Result is 2. Failed to identify address for specified name 'www.cisco.com.', requested addr type 2.
***** FATAL ERROR ***** Reporting Task: DNSC. [debug data] ***** END OF FATAL ERROR *****
Administrators reporting the issue on platforms including Reddit, Cisco Community forums [1, 2], and direct communications with BleepingComputer confirmed that the errors originate from the DNSC (DNS Client) task. They typically occur when switches attempt to resolve “www.cisco.com” or synchronize time via NTP servers.
Network operators described the reboot cycle as occurring every few minutes, creating severe disruptions. One Reddit user stated, “The cycle repeats every few minutes. This is obviously pretty disruptive and I’m not going to be able to sustain operations like this for very long.”
The bug reportedly affects multiple Cisco switch models, including:
- Cisco CBS250 series
- Cisco CBS350 series (including CBS350-24P-4G)
- Cisco Catalyst C1200 series
- Cisco SG350
- Cisco SG350X
- Cisco SG550X series
Reports indicate that failures began nearly simultaneously across separate networks, suggesting a global trigger or a time-based condition.
Although Cisco has not publicly released the root cause, BleepingComputer learned that Cisco support acknowledged the issue for at least one customer, confirming it impacts CBS, SG, and Catalyst 1200/1300 series switches.
In the meantime, network administrators have implemented temporary workarounds to stop the reboot loops. These include disabling DNS resolution, turning off SNTP/time synchronization, and blocking outbound internet access from switch management interfaces.
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