High CPU Usage Due to Excessive Process Switching
I was called in to troubleshoot a router that was behaving sluggishly—routing updates were delayed, SSH sessions kept timing out, and SNMP monitoring was erratic. The router wasn’t crashing, but it felt like it was gasping for air. A quick check revealed CPU usage hovering around 90–100%, even during off-peak hours.
🔍 Diagnosis
I ran:
show processes cpu sorted— this showed that the IP Input process was consuming most of the CPU.show ip traffic— revealed a high number of packets per second, especially small packets.show interfaces— confirmed that the router was receiving traffic at a high rate, but not enough to justify the CPU spike.
That’s when I realized the router was process-switching packets instead of using CEF (Cisco Express Forwarding)—a much more efficient method.
✅ Solution
Here’s how I resolved it:
- Enabled CEF globally:
- Verified CEF on interfaces:
- If CEF wasn’t enabled per interface, I added:
- Checked for ACLs or features forcing process switching: Some legacy ACLs were applied in a way that bypassed CEF. I restructured them using optimized match conditions and reordered them to reduce CPU load.
- Monitored CPU again:
conf t ip cef
show ip interface GigabitEthernet0/1
interface GigabitEthernet0/1 ip route-cache cef
show processes cpu history
📚 Reference That Helped
The Cisco Enterprise Troubleshooting Guide was instrumental. It explained how process switching can overwhelm routers and how enabling CEF can drastically improve performance. I also found practical tips on identifying CPU hogs and optimizing ACLs.
This issue is common in older configurations or when routers are upgraded but legacy settings remain. It’s a great reminder that performance isn’t just about bandwidth—it’s about how efficiently the router handles packets.
Would you like me to help you turn this into a visual performance optimization checklist or add it to your training materials?
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