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Cisco Bug: CSCvh63646 - False positive duplicate ARP message for idle clients during race condition

Last Modified

Aug 19, 2019

Products (1)

  • Cisco 5500 Series Wireless Controllers

Known Affected Releases

8.2(166.0) 8.3(133.0) 8.5(110.0) 8.6(101.1)

Description (partial)

Symptom:
On a given  scenario where we have a long number of devices connecting dynamically to the network, i.e. public free wifi, and with the need of short DHCP lease times (2 hours i.e.) we may fail into a condition where lease times out on client but WLC WLAN session is still alive depending on WLAN session timeout. Since device, i.e. Macbook, sends null frames over the air while sleep and WLC wouldn't erase the client entry from database or ARPT table until the session times out.

Then, after 2 hours when a new device arrives and attempts to get IP address, since lease time has expired for DHCP, the server may assign the already available IP address on a valid wat to a new device. The IP would no be used anymore by sleeping device apparently but still active on WLCs client database/ARP table. When WLC receives DHCP information (to glean IP information to ARP table and client's database), WLC will show next false positive message (Expecting that mobile device will honor the DHCP lease time and ask for a different or the same IP via DHCP after waking up before using it):

DHCP Socket Task: Timestamp: %DTL-3-ARP_CLIENT_IP_DUPLICATED: [PA]dtl_arp.c:1888 ARP entry overwrite, conflict detected via ARP Request from client with MAC-ID  xx:xx:xx:xx:xx:xx and IP Address  10.10.10.10, Old client MAC-ID was yy:yy:yy:yy:yy:yy

This is a race condition scenario and a problem with false positive log, at the end, new device should get IP address and connect successfully to the network.

Conditions:
High DHCP demand for addresses
Short DHCP lease time < WLAN session timeout
Mobile device sending null frames while sleep. i.e. Macbook
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