Plain old IOS is a monolithic program and tends to be used in older routers and current small-to-medium routers & modern spins of older product lines. IOS XR r

Plain old IOS is a monolithic program and tends to be used in older routers and current small-to-medium routers & modern spins of older product lines. IOS XR runs on QNX and tends to be used on routers in a carrier role. IOS XE runs on Linux and tends to be used on service/aggregation routers.