Brocade FastIron Ethernet Switch Traffic Management Guide - Manual (Page 48)

specify in the policy, but drops additional bytes (packets in the case of ICX 6650). Unused bandwidth
is not carried over from one interval to the next.
NOTE
Port based Rate Limiting affects only known-unicast traffic. Broadcast, Multicast and Unknown-unicast
(BUM) is not affected by this rate. To rate limit the BUM traffic, use BUM rate limiting as described in
chapter BUM Rate Limiting.
NOTE
Brocade recommends that you do not use Fixed rate limiting on ports that receive route control traffic
or Spanning Tree Protocol (STP) control traffic. If the port drops control packets due to the Fixed rate
limiting policy, routing or STP can be disrupted.
Rate limiting applies to inbound ports and rate shaping applies to outbound ports.
Rate limiting in hardware
Each Brocade device supports line-rate rate limiting in hardware. The device creates entries in
Content Addressable Memory (CAM) for the rate limiting policies. The CAM entries enable the device
to perform the rate limiting in hardware instead of sending the traffic to the CPU. The device sends the
first packet in a given traffic flow to the CPU, which creates a CAM entry for the traffic flow. A CAM
entry consists of the source and destination addresses of the traffic. The device uses the CAM entry
for rate limiting all the traffic within the same flow. A rate limiting CAM entry remains in the CAM for
two minutes before aging out.
How Fixed rate limiting works
Fixed rate limiting counts the number of bytes (packets in ICX 6650) that a port receives, in one
second intervals. If the number exceeds the maximum number you specify when you configure the
rate, the port drops all further inbound packets for the duration of the one-second interval.
Once the one-second interval is complete, the port clears the counter and re-enables traffic.
The following figure shows an example of how Fixed rate limiting works. In this example, a Fixed rate
limiting policy is applied to a port to limit the inbound traffic to 500000 bits (62500 bytes) (packets in
ICX 6650) a second. During the first two one-second intervals, the port receives less than 500000 bits
Rate limiting in hardware
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