In task 3.1 it says to compress the HDLC header, but the command "compress stac" only compresses the L2 payload.
Layer 2 payload compression involves
the compression of the payload of a Layer 2 WAN protocol, such as PPP,
Frame Relay, High-Level Data Link Control (HDLC), X.25, and Link Access
Procedure, Balanced (LAPB). The Layer 2 header is untouched by the act
of compression. However, the entire contents of the payload (that
include higher-layer protocol headers) are compressed. They are
compressed as described in How does data compression work?, and use either a form of the "stacker" algorithm (based on the industry standard Lemple Ziv algorithm; refer to the American National Standards Institute (ANSI) document X3.241-1994), or the "predictor" algorithm, which is an older algorithm that is mostly used in legacy configurations.