Fast Path QoS - Exception Rate Limitation is a feature of Fast Path QoS that limits the rate of packets sent as exceptions from the fast path to Linux.
- Exception Rate Limitation to limit the rate of packets sent as exceptions from the fast path to Linux
When the MCORE_TC_ERL option is enabled, the fast path limits the rate of exceptions sent to the slow path. The rate can be configured by “fp-cli tc-erl-add” command.
A priority is associated with each exception: low, medium and high. The pass or drop action depends on this priority and the packet color, as marked by the QoS marker:
- If exception priority is low, green packets pass and yellow or red packets are dropped,
- If exception priority is medium, green and yellow packets pass, and red packets are dropped,
- If exception priority is high, packets pass,
- If exception priority is unknown, packets are dropped.
Whenever a packet is found to be an exception, a class value (8 bits) is associated with it:
- The two most significant bits define the priority of the exception (3 for high priority, 2 for medium, 1 for low, and 0 for unknown),
- The six last significant bits define the type of the exception (ARP is required; IKE negotiation is needed, etc.).
The default setting is low priority for all exceptions type, so that a simple token bucket is used to let packets go through or to drop them.