High CPU Usage Due to Excessive Process Switching

HIGH CPU USAGE

I recently had to troubleshoot a router that was acting sluggish. Routing updates lagged, SSH sessions kept dropping, and monitoring tools weren’t giving consistent results. The device wasn’t crashing outright, but it felt like it was constantly struggling to keep up. A quick check showed CPU utilization stuck around 90–100%, even when traffic was light.

🔍 What I Found

• Running a CPU process check revealed one process dominating usage.

• Traffic analysis showed a flood of small packets per second.

• Interface stats confirmed incoming traffic was steady but not high enough to justify the CPU spike.

That’s when it clicked: the router was handling packets inefficiently by process-switching instead of leveraging a faster forwarding method.

✅ How I Fixed It

1. Enabled efficient forwarding globally I turned on the optimized forwarding feature (similar to CEF in Cisco devices).

   Code

   conf t
   ip cef
   

2. Verified interface settings Checked each interface to confirm the forwarding method was active.

   Code

   show ip interface GigabitEthernet0/1
   

3. Adjusted configurations

   • Enabled route caching where needed.

   • Reviewed access lists and restructured them so they wouldn’t force packets back into process switching.

4. Monitored results After the changes, CPU usage dropped to under 30%, and the router became responsive again.

📌 Key Takeaway

This issue often shows up in older setups or when legacy configurations remain after upgrades. It’s a reminder that performance isn’t just about bandwidth—it’s about how efficiently packets are handled. Keeping forwarding optimized and ACLs streamlined can prevent routers from choking on unnecessary CPU load.