After receiving an ICMP Request packet, a switch does not send it to the ICMP protocol stack but directly returns an ICMP Reply message. This process is called fast ICMPM reply. Switches cannot accurately calculate the delay or jitter of ping packets that require high real-time performance. The protocol stack adds the sending and receiving timestamps to ping packets. The packets need to enqueue frequently between the protocol stack and hardware. This task switching cannot ensure real-time performance of timestamps.

In versions earlier than V100R006, fast ICMP reply is a common task. When a switch processes a large volume of service traffic, the reply delay is long due to task switching. In V100R006 and later versions, fast ICMP reply is a super task with a high priority. In most cases, the reply delay is about 1 ms. The actual delay depends on the CPU usage and is shorter than 100 ms.

 NOTE:

By default, box switches have fast ICMP reply enabled in all versions.