Administrator Guide

Layer 2 Switching Commands 399
rate-limit cpu
Use the rate-limit cpu command to reduce the amount of unknown
unicast/multicast packets forwarded to the CPU. Use the no form of the
command to set the rate limit to the default value.
Syntax
rate-limit cpu direction input pps pps_value
no rate-limit cpu direction input pps
• pps_value—The packets per second. The range is 100-1024 packets per
second (100-3000 packets per second for N4000 series switches).
Default Configuration
The default ingress rate limit is 1024 packets per second (3000 for N4000
series switches ).
Command Modes
Global Configuration mode
User Guidelines
Unknown unicast and multicast packets are copied to the CPU on the lowest
priority QoS queue. Unknown packets are those that do not have hardware
forwarding entries. Known unicast/multicast packets are hardware forwarded
and are not queued to the CPU. Control plane packets (e.g. spanning tree
BPDUs) are copied or forwarded to the CPU on higher priority queues. The
rate limiting for unknown packets occurs on the internal CPU port and does
not affect hardware based traffic routing/forwarding in any way. Typically, the
switch examines the received packets in software to check if there is a
forwarding entry, create a forwarding entry (e.g., add a L2 MAC address or
ARP response), and then either discard the packet or software forward the
packet (only occurs during the brief transitional period when the system is
actively adding a hardware forwarding entry but the hardware is not yet
updated). Processing delays for higher priority packets may occur when the
internal CPU queue is continually kept busy handling low priority packets.