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.

Other related questions:
Causes for a long period of obtaining an IP address on an S series switch
For S series switches excluding the S1700, the main cause is that the STP function is enabled. (By default, STP on all switch interfaces is enabled.) When the interface changes from Down to Up, STP convergence is performed (the process takes about 30s). During STP convergence, a switch interface directly discards received packets. That is, DHCP Request messages are all discarded during the STP convergence period. As a result, IP addresses are obtained slowly in this case. Use either of the following method: - Run the stp edged-port enable command on the switch interface connected to a user terminal to configure the interface as an edge interface. If the switch is configured with the stp bpdu-protection command, the edge interface of the switch becomes Down when there are malicious attacks of BPDUs. To enable the edge interface to automatically go Up, run the error-down auto-recovery cause bpdu-protection interval interval-value command to configure the interface to restore Up state automatically. - If there is no physical loop, run the stp disable command to disable STP on the switch or interfaces connected to users. In another scenario where the switch functions as a DHCP server, a user obtains the IP address for more than 1 minute when the network cable of the PC is removed and reinstalled. This problem occurs due to a software bug. When the network cable of the PC is removed and reinstalled, the PC sends DHCP Request messages to request for an IP address. In earlier versions of switches, some problems occur in the software.

The reason of S series switch Ping packet loss
For S-series chassis switches: The Ping packets of the local switch are processed by the switch as a fib-hit packet. For packets of type fib-hit, the switch sends to CPUs at the default CAR value to prevent this type of packets from impacting the CPU. In the case of the default CAR value, the number of ping packets per second is too large for the CAR value, so the switch will discard some packets. Increase the fib value of fib-hit, you can solve the packet loss problem.

How long is the NTP synchronization interval on an S series switch
The maximum NTP synchronization interval on an S series switch (except the S1700) is the tenth power of 2 in seconds, namely, 1024 seconds. The interval for clock synchronization between the NTP client and server is not fixed and depends on the clock precision. A higher clock precision results in a longer synchronization interval. The synchronization interval depends on the following two factors: 1. Time difference between the client and server 2. Frequency offset of the clock on the client compared with the clock on the server

Intermittent ping interruption on S series switch
If an S series switch fails to be pinged intermittently, check whether ICMP packets are discarded by CPCAR. - If ICMP packets are discarded by CPCAR, ping packets cannot be sent to the CPU. In this case, modify the CPCAR value of ICMP packets. Note: An improper CPCAR setting will affect services on your network. It is recommended that you contact Huawei technical support personnel before adjusting the CPCAR setting. -If ICMP packets are not discarded by CPCAR, run the undo icmp rate-limit enable command to disable ICMP rate limit. Then check whether forwarding information for ping packets, such as the returned route, exist on the switch.

Many STAs connected to APs have a latency
If the connected STAs have a latency, use tool software to collect the environment information and perform comparison tests to troubleshoot the problem. The possible causes of this problem are as follows: - The user access rate is low. - The WLAN environment is poor and much interference exists. - Too many users are connected. - Too many low access rate users are connected, deteriorating the network performance. - The cable network is of poor quality. - The NICs of STAs are of poor performance.

If you have more questions, you can seek help from following ways:
To iKnow To Live Chat
Scroll to top