We have recently purchased a series of 2800 series routers. They all came with SDM preinstalled. However, two of them choose to completely ignore the sdm default configuration file (sdmconfig-2801.cfg). SDM works OK, as long as I configure the routers by hand for remote HTTP/HTTPS access.
The routers seem identical in every way - but all the others load the sdm config file (with the banner, the username/password, the http access class, and so on).
Is there any hidden setting that determines whether a router loads that configuration file or not?