Router memory and resource constraints prevent a router from loading all Cisco IOS IPS signatures. Thus, it is recommended that you load only a selected set of signatures that are defined by the categories. Because the categories are applied in a "top-down" order, you should first retire all signatures, followed by "unretiring" specific categories. Retiring signatures enables the router to load information for all signatures, but the router will not build the parallel scanning data structure.
Retired signatures are not scanned by Cisco IOS IPS, so they will not fire alarms. If a signature is irrelevant to your network or if you want to save router memory, you should retire signatures, as appropriate
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