IOS IPS customers running version 12.4T, 15.0M, or 15.1M - a critical software defect has been identified which may cause your router to reload and be stuck in a boot loop if IOS IPS signature version S639 or later is installed on the device. Recovery of impacted devices is possible only via a serial console connection through the device's ROMMON mode. For customers who are using IOS IPS signatures S638 or earlier, there is no issue. Customers wishing to upgrade the IOS IPS signature version to S639 or later must first be running a fixed version of IOS on the device prior to upgrading the IPS signatures. Fixed versions of IOS include: 15.2(4)M, 15.1(3)T4, 15.2(3)T1, 15.1(4)M5, 12.4(24)T8 and later. Please refer to defect CSCtz27137 for additional details and steps to recover impacted devices. If you need further assistance please contact Cisco TAC.