Fast ICMP reply function on S series switches

After receiving an ICMP Request packet, an S series switch (a non-S1700 switch) does not send it to the ICMP protocol stack but directly exchanges the source and destination MAC addresses of the packet and responds with an ICMP Relay message. This process is called fast ICMP reply. (The switch can successfully ping another device even if it does not have a return route or an ARP entry.) A switch cannot accurately calculate the delay or jitter of ping packets that require high real-time performance. The reason is that the protocol stack adds the sending and receiving timestamps to ping packets. The packets need to enter queues frequently between the protocol stack and the hardware. Therefore, the real-time performance of timestamps cannot be ensured.
In versions earlier than V100R006, fast ICMP reply is a common task. When a switch processes a large number of services, the reply delay is long due to service switching. In V100R006 and later versions, fast ICMP reply is a super task with a high priority. In most cases, the relay delay is about 1 ms. The actual delay depends on the CPU usage and is shorter than 100 ms.
The icmp-reply fast command enables the fast ICMP reply function.
Note: The fast ICMP reply function is disabled on modular switches running all versions by default.

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