Outposts, not permenant military installations but small semi-autoninous camps that receive orders from superiors in Raven Rock. In any event hardly enough people to continue to exist without some serious interbreeding. Besides, the state of Illinois is largely patrolled and controlled by the http://fallout.wikia.com/wiki/Midwestern_Brotherhood_of_Steel.
Unfortunately, the Midwestern Brotherhood of Steel was featured in Fallout Tactics, which is not considered canon. According to Fallout 3, only a small, rogue detachment of the Brotherhood of Steel survives in Illinois, not the semi-autonomous empire consisting of humans, deathclaws, mutants, and ghouls, led by airship-crash-survivors, that one can create when playing the game. This would mean, in canon the Brotherhood has one small outpost, compared with the "small semi-autonomous camps" you would say the Enclave controls from the word 'outposts.' So, the Brotherhood has one, "small semi-autonomous camp."
P.S.
For those who don't know, Fallout Tactics: Brotherhood of Steel contains many an innaccuracy and departure from the Fallout series.
Just from the wikia:
-The setting is not retro 1950s, but instead a mix of modern and world war II tech,
-the Brotherhood's origin story is completely changed and retconned (later retconned back to the original in Fallout 3, casting more doubt on anything from this game, like the Midwest BOS),
-Fossile Fuels are plentiful, even for raiders, along with gas-powered vehicles, meaning the Resource Wars didn't happen. This subsequently means the Great War should not have happened
-The Brotherhood can easily manufacture new power-armor suits, rather than simply maintain them as canon says they do
-Vault 0 is not designed like a Vault Experiment Vault
-Ghouls suffer Rad Poisoning