Intermittent packet drops can occur for various reasons. Go through the performance troubleshooting steps to be sure you are not hitting any performance issue. In addition, you may need to check the following to troubleshoot the intermittent packet drops issue:
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Fragmentation issue If you are running a version of IOS code older than 12.3(8) T, you may run into issues with fragmentation. CBAC does not support IP fragments earlier than version 12.3(8) T, so packets may be dropped intermittently if they come out of order. To get around this problem, you may configure IP TCP adjust-mss under the interface facing towards the client, so that MTU negotiation can take place, and both client and server adjust their MTU to avoid the fragmentation.
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URL filtering If you are running IP URL filtering, be sure that the policy defined in the URL server is not denying the packet.