For which router configuration is the attack-drop.sdf file recommended?

A.
Routers with less than 128 MB of memory.

B.
Routers with less than 64 MB of memory.

C.
Routers with at least 128 MB of memory.

D.
Routers with at least 192 MB of memory.

E.
Routers with at least 256 MB of memory.

Explanation:
An SDF has definitions for each signature it contains. After signatures are loaded and
complied onto a router running Cisco IOS IPS, IPS can begin detecting the new
signatures immediately. If the default, built-in signatures that are shipped with the
routers are not used, then one of three different types of SDFs can be selected for
download, which are pre-configured for routers with memory requirements:
 attack-drop.sdf file (which is a static file that has 83 signatures) is used for
routers with less than 128MB memory.
 128MB.sdf (which has about 300 signatures) is used for routers with 128 MB or
more memory.
 256MB.sdf (which has about 500 signatures) is used for routers with 256 MB or
more memory.
http://www.cisco.com/c/en/us/td/docs/iosxml/ios/sec_data_ios_ips/configuration/12-4t/sec-data-ios-ips-12-4t-book/sec-cfgips.html#GUID-09308574-4A31-4DBF-820E-A3F03BC47512