Wireless Terminals in a College Cannot Ping the Gateway

Publication Date:  2015-06-16 Views:  169 Downloads:  0
Issue Description
On a college network, some laptops manufactured in 2005 or 2006 cannot ping the gateway and TSM server after associating with an 802.11bgn AP. After the radio mode of the AP is changed from 802.11bgn to 802.11gn or 802.11bg, the fault is rectified.

The network diagram is as follows:



Involved Products and Versions

Huawei APs of V200R003C00SPC300 and earlier versions, such as AP6010 and AP7110.

Handling Process
Perform packet header obtainment. The analysis of the obtained packets shows that the STA sends an ICMP Request packet, and the gateway responds with an ICMP Reply packet; however, the Reply packet is discarded by the core switch due to lack of ARP entry for the STA. The core switch sends out an ARP packet. The AP receives and sends out the ARP packet, but the STA does not receive it. Based on the analysis, the fault occurs on the wireless port. Check the ARP packet rate. It is found that the rate of the ARP packet sent from the AP to the laptop is 6.5 Mbit/s (one of the rates supported in 802.11n radio mode), which is not supported by the legacy terminals.

Root Cause
The legacy terminals do not support the ARP packet rate and radio mode of the 802.11bgn AP.

Solution
Change the radio mode of the AP to 802.11bg and deliver the configuration.

Suggestions
The fault occurs because the legacy terminals do not support the 802.11bgn mode. After the radio mode is changed to 802.11bg, the fault is rectified.

The ping method is a very useful in fault troubleshooting. If a ping operation fails, a request packet is not sent out or a reply packet is not received. When you ping an IP address, the device first checks whether the ARP entry mapping the IP address exists in the ARP table. If so, the device sends out an ICMP Request packet. If not, the device sends out an ARP Request packet to create the corresponding ARP entry and then sends an ICMP Request packet.

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