DNS client failure sends small business networks into repeated reboot loops
San Jose, California, 9 January 2026 US – Network administrators across the world faced unexpected outages this week after thousands of Cisco small business switches began crashing and rebooting repeatedly. The issue, first reported in the early hours of January 8, 2026, was traced back to a critical failure in the DNS client service running on several popular switch models.
The affected devices entered reboot loops every few minutes, bringing business operations to a halt until administrators intervened manually. According to user reports, the problem began around 2:00 AM UTC and spread rapidly across different regions, hitting multiple Cisco switch families at the same time.
What caused the crashes
The issue impacted several models, including CBS250, CBS350, C1200, SG350, and SG550X series switches from Cisco. System logs from affected devices showed fatal DNS client errors, specifically DNS_CLIENT-F-SRCADDRFAIL messages. These errors occurred when switches failed to resolve common domain names such as [www.cisco.com](http://www.cisco.com) and official NIST time servers like time-c.timefreq.bldrdoc.gov.
Once the DNS lookup failed, the DNS client task, known as DNSC, triggered a fatal error. This caused the switch to generate a core dump and automatically reboot. In many cases, the device would restart successfully, only to crash again minutes later when the same DNS process ran again.
Administrators confirmed the crashes were happening on multiple firmware versions, including 4.1.7.17, 4.1.3.36, and 4.1.7.24. This suggests the problem is not limited to a single software release but is rooted deeper in how the DNS client handles failures.
Real-world impact on businesses
Reports flooded Cisco’s community forums and Reddit as administrators struggled to stabilize their networks. Many said they were managing dozens of affected switches across offices, warehouses, and retail locations.
One network administrator reported that all 50 of their CBS250 and C1200 switches crashed on the same day and only stabilized after DNS settings were removed. Others managing SG550X switches confirmed identical behavior starting at the exact same time across different sites.
In several cases, the switches attempted DNS lookups even when administrators had not manually configured NTP services. The devices were still trying to reach default SNTP servers such as time-pnp.cisco.com, which triggered the fatal DNS errors.
Why DNS made the problem worse
Forum users suspect the issue may have been amplified by a resolver-side change involving Cloudflare’s 1.1.1.1 DNS service. Some administrators observed that networks using only Cloudflare DNS were more likely to experience the crash, while those with secondary resolvers such as Google’s 8.8.8.8 appeared less affected.
At the core of the problem is how Cisco’s DNS client handles failures. Instead of retrying or timing out safely, the client treats certain DNS lookup failures as fatal errors. This lack of resilience turned a routine DNS issue into a denial-of-service–like event for small business networks.
Temporary fixes and workarounds
Until an official patch is released, administrators have been forced to rely on manual workarounds to keep their switches running. The most effective steps include:
Disabling DNS services using commands such as no ip name-server and no ip domain-lookup.
Removing default SNTP servers like time-pnp.cisco.com from switch configurations.
Blocking outbound internet access from switches to prevent external DNS lookups.
After applying these changes, most administrators reported that their switches stabilized immediately. However, disabling DNS also removes hostname resolution, which can make configurations and troubleshooting more difficult.
Cisco response and broader concerns
Cisco support teams have acknowledged the issue to customers and confirmed that it affects CBS, SG, and Catalyst 1200 and 1300 series switches. As of January 9, 2026, there is no public security advisory, field notice, or firmware update addressing the problem.
The incident has raised concerns among IT professionals about the reliability of small business networking equipment when faced with external service changes. A simple DNS failure should not be able to take down core network infrastructure, administrators argue.