Core issue
This issue can be caused by any of these situations:
- Generic Routing Encapsulation (GRE) tunnels are not supported on the Cisco Catalyst 3550 Switch.
The reason for this is that the Cisco Catalyst 3550 Switch uses hardware-based Cisco Express Forwarding (CEF) switching. There is no way to CEF-switch GRE packets. GRE packets must be encapsulated by the software. The hardware does not have the capability to encapsulate the packets. Consequently, this traffic is processed or software switched. The software traffic can quickly cause the CPU to spike.
- An extended ping from one interface to another interface on the same switch can cause high CPU utilization.
This can occur when a large number of ping packets are sent and received. This is the expected behavior.
On the Catalyst 3550 Switch, Layer 3 forwarding of IPv4 in the Subnetwork Access Protocol (SNAP) can only be done in the software. SNAP-encapsulated IPv4 packets that are directed to the router MAC address or the Hot Standby Router Protocol (HSRP) group MAC address (if this is the active router in the HSRP group) are forwarded to the switch CPU. This action can potentially cause high CPU utilization levels.
Packets received from media types that require SNAP encapsulation of IPv4 packets require the switch to forward SNAP-encapsulated packets. Ordinarily, Layer 2 forwarding of IPv4 in SNAP encapsulation takes place in the hardware (unless a VLAN map or port Access Control List [ACL] contains an IP ACL). However, this cannot take place on the Cisco Catalyst 3550 Switch.
High CPU utilization due to the IP input process The IP input process takes care of process-switching IP packets. If the IP input process uses unusually high CPU resources, the router is process-switching a lot of IP traffic.
This can happen when you use Policy Based Routing (PBR) with deny statements in its ACL. Traffic hitting these deny statements is forwarded in the software. This is not scalable because the Catalyst 3550 Switch forwards the packets by using the hardware.
Resolution
In order to resolve high CPU utilization issues due to the IP input process, modify the ACLs to use Permit statements.
Additional information can be found in the IP Input section of Troubleshooting High CPU Utilization in IP Input Process.
For more information on the limitations and restrictions of features which can cause this issue, refer to Release Notes for the Catalyst 3550 Multilayer Switch.
