Long ping latency on S series switches

Network latency indicates the round-trip period of time during which a source device sends a packet to the destination device and then the destination device returns a packet to the source device.
Possible causes of long network latency are as follows:
1. Multiple hops on the packet forwarding path. The transmission time of packets in the physical medium can be ignored because optical and electrical signals are transmitted at a high speed. However, the time that a switch spends processing packets cannot be ignored. When packets are transmitted through too many hops, the network latency is long.
2. Insufficient network bandwidth. When the network through which packets are transmitted does not have sufficient bandwidth, network congestion occurs and packets need to wait in queues, resulting in long network latency.
3. Insufficient memory space. When a switch receives a large number of packets, the switch does not have sufficient memory space to process these packets, resulting in slow packet processing speed and long network latency.

You can run the ping command to test network latency. The test results are only for reference and cannot be used as an absolute value of network latency measurement. No reference value is available for determining whether the ping latency is normal because requirement for network latency varies depending on network status. Other measurement methods such as network quality analysis (NQA) are also required to accurately measure network latency. Pay attention to the following points when analyzing a ping latency:
1. When a switch forwards packets through the hardware at a high speed, network latency is short. For example, ping a PC connected to the switch. When packets need to be processed by the CPU, network latency is long.
For example, ping a gateway. Through network latency is long when the switch pings the gateway, packets are normally forwarded because the packets are processed by the underlying chip rather than the CPU. You can run the icmp-reply fast command to enable the fast ICMP reply function on the switch to shorten network latency when the switch pings the gateway. After the function is enabled, the switch quickly responds to received Echo Request packets destined for its own IP address. The CPU of the LPU directly responds to the received ICMP packets, improving the processing speed of ICMP packets and shortening network latency.
2. The processing priority of ICMP packets has been minimized to prevent impacts of common ping attacks on the switch, so that ICMP packets are the last to be transmitted and processed. Therefore, the network latency is long.

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