In some network applications, a high CPU usage is a normal situation. Generally, a large network requires more CPU resources to process network traffic. When more member switches need to be managed in a stack, more CPU resources are required to maintain and manage the stack.

The following is list of scenarios where CPU usage commonly becomes high, but should not be considered a fault.

• Spanning tree

  In MSTP, CPU usage is directly proportional to the number of instances and active ports. In VBST, each VLAN runs an independent instance. Therefore, VBST uses more CPU resources than MSTP when VBST and MSTP have the same number of VLANs and ports.

• Routing table update

  When a Layer 3 switch receives a route update message, the switch uses CPU resources to update routing information to the forwarding plane. In a stack, the switch also needs to synchronize routing information to other member switches. During a routing table update, the following factors affect CPU usage:

  • Number of routing entries
  • Update frequency
  • Number of routing protocol processes that receive the update message
  • Number of member switches in a stack

• Command execution

  CPU usage temporarily becomes high when commands that have a long execution time are used, for example:

  • The copy flash:/ command is executed in the user view.
  • Some debugging commands have a large amount of display information, especially when debugging information is displayed through the serial port.

• Other scenarios

  • A port fast learns MAC addresses after having the sticky MAC function enabled.
  • A large number of ports are added to a large number of VLANs. For example, port groups are used to add a large number of ports to a large number of VLANs and change the link type of these ports.
  • Frequent or a large number of IGMP requests
  • Frequent network management operations
  • A large number of concurrent DHCP requests (For example, when a switch functions as a DHCP server, it restores connections with a large number of users.)
  • ARP broadcast storm
  • Ethernet broadcast storm
  • A large number of concurrent protocol packets are forwarded through software. For example, L2PT transparently transmits a large number of BPDUs within a short time or DHCP relay/snooping-enabled switch forwards DHCP packets through software.
  • A large number of data packets that cannot be forwarded through hardware are sent to the CPU, such as ARP Miss packets.
  • A port frequently alternates between Up and Down states.