With IPHost Network Monitor you can run simple snmp requests against a Cisco device in your network.
mnRepliesDroppedInvalidExtension
mn Replies Dropped Invalid Extension
1.3.6.1.2.1.44.1.3.3.10
Total number of Registration Replies dropped by the mobile node due to both poorly formed extensions and unrecognized extensions with extension number in the range 0-127.
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IPHost Network monitor allows you to monitor mnRepliesDroppedInvalidExtension on Cisco device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Cisco wireless switches right now.