Global DNS Outage Disrupts Cisco Small Business Switches
On January 8, 2026, network administrators worldwide encountered significant disruptions as numerous Cisco small business switches experienced continuous reboot cycles. The root cause was identified as fatal errors within the devices’ DNS client services, leading to operational instability.
Incident Overview
The issue emerged around 2:00 AM UTC, predominantly affecting models such as the CBS250, C1200, CBS350, SG350, and SG550X series switches. Administrators observed that these devices entered reboot loops every few minutes, severely impacting network operations. The problem persisted until DNS configurations were manually removed from the affected devices.
Technical Details
System logs from the impacted switches revealed recurring DNS_CLIENT-F-SRCADDRFAIL errors. These errors indicated failures in resolving domain names like www.cisco.com and NIST time servers such as time-c.timefreq.bldrdoc.gov. The DNS client task (DNSC) encountered fatal errors, resulting in core dumps and automatic device resets. Stack traces pointed to DNS resolution failures in firmware versions including 4.1.7.17, 4.1.3.36, and 4.1.7.24.
Community Reports
Administrators managing multiple affected devices reported widespread issues. One user noted, Every single one crashed today… until I removed the DNS configuration, referencing 50 CBS250 and C1200 units. Similar reports surfaced on Reddit, with SG550X owners confirming identical symptoms across various sites.
Affected Software Versions
The following table summarizes the affected models and their corresponding firmware versions:
| Model/Series | Reported Versions | Date Codes |
|————–|——————-|————|
| CBS250/C1200 | 4.1.7.17, 4.1.3.36 | May 2025, May 2024 |
| CBS350 | 4.1.7.24, 3.5.3.2 | Aug 2025, Unknown |
| SG550X | Various recent | N/A |
Root Cause Analysis
The crashes were linked to DNS lookups for default SNTP servers like time-pnp.cisco.com or www.cisco.com, even on switches without explicit NTP configurations. Some forum users speculated that changes on Cloudflare’s 1.1.1.1 DNS resolver might have exacerbated the issue, as secondary servers like 8.8.8.8 appeared to mitigate the problem. Cisco’s DNS client treating lookup failures as fatal contributed to the devices’ instability.
Recommended Workarounds
To stabilize the affected switches, administrators implemented the following workarounds:
– Disable DNS: Execute the commands `no ip name-server` and `no ip domain-lookup` to prevent the switch from performing DNS lookups.
– Remove Default SNTP Server: Use the command `no sntp server time-pnp.cisco.com` to eliminate reliance on potentially problematic time servers.
– Restrict Internet Access: Implement access control lists (ACLs) to block outbound internet access from the switches, reducing exposure to external DNS issues.
While these measures restored stability, disabling DNS may limit the ability to resolve hostnames within device configurations.
Cisco’s Response
Cisco support acknowledged the problem, confirming impacts on CBS, SG, and Catalyst 1200/1300 series switches. As of January 9, 2026, no public advisory or patch has been released. Administrators are advised to monitor Cisco’s official channels for updates and apply the recommended workarounds to maintain network stability.
Implications for Small Business Networks
This incident underscores the vulnerability of small business networks to disruptions stemming from routine DNS issues. The synchronized onset of the problem suggests a global trigger, possibly related to external DNS changes, highlighting the need for robust firmware resilience in embedded systems.
Recommendations for Network Administrators
– Monitor for Updates: Regularly check Cisco’s official communications for firmware updates or patches addressing this issue.
– Implement Workarounds: Apply the recommended configurations to mitigate the immediate impact on network operations.
– Review Network Configurations: Assess the reliance on external DNS services and consider implementing internal DNS solutions to reduce dependency on external factors.
– Enhance Monitoring: Deploy monitoring tools to detect unusual patterns in DNS resolution and device behavior, enabling proactive responses to potential issues.
Conclusion
The global DNS outage affecting Cisco small business switches highlights the critical importance of resilient network configurations and proactive management. By implementing the recommended workarounds and staying informed about firmware updates, network administrators can mitigate the impact of such incidents and maintain operational stability.
