diff --git a/doc/stp.md b/doc/stp.md
index 0ccca7c..49bedd3 100644
--- a/doc/stp.md
+++ b/doc/stp.md
@@ -59,9 +59,10 @@ to the CPU. `stp_setup()` prints a warning for every STP-enabled port in that
 state.
 
 Port states live in `RTL837X_MSTP_STATES (0x5310)`, two bits per port:
-`00` disabled, `01` blocking, `10` learning, `11` forwarding. In the blocking
-state a port forwards nothing except frames sent by the CPU, and nothing it
-receives reaches the CPU.
+`00` disabled, `01` blocking, `10` learning, `11` forwarding. A port in
+blocking forwards nothing between ports, but it still sends what the CPU
+hands it and still passes a received BPDU up to the CPU, which is what lets
+loop detection go on working on a port it has already blocked.
 
 ## Timers
 
diff --git a/rtl837x_common.h b/rtl837x_common.h
index 50bffb1..4dcadce 100644
--- a/rtl837x_common.h
+++ b/rtl837x_common.h
@@ -72,9 +72,8 @@ struct vlan_tag {
 #define RTL_FRAME_TAG_ID	0x8899
 #define RTL_FRAME_TAG_VERSION	0x04
 /* Bits of the tag's `flags` word, see doc/CpuPort.md. */
-#define RTL_TAG_LEARN_DIS	0x0020	/* do not learn the CPU's SA on the egress port */
+#define RTL_TAG_LEARN_DIS	0x0020	/* do not learn the source address from this frame */
 #define RTL_TAG_KEEP		0x0080	/* keep the frame's 802.1Q tag format as injected */
-/* The `pmask` word, see doc/CpuPort.md. */
 
 // For TX, an 8 byte (plus 4 byte padding when when VLAN is enabled)
 // header describing the frame to be moved to the Asic is used