Index: head/sys/dev/iwn/if_iwn.c
===================================================================
--- head/sys/dev/iwn/if_iwn.c
+++ head/sys/dev/iwn/if_iwn.c
@@ -1351,6 +1351,8 @@
 	ivp->iv_newstate = vap->iv_newstate;
 	vap->iv_newstate = iwn_newstate;
 	sc->ivap[IWN_RXON_BSS_CTX] = vap;
+	vap->iv_ampdu_rxmax = IEEE80211_HTCAP_MAXRXAMPDU_64K;
+	vap->iv_ampdu_density = IEEE80211_HTCAP_MPDUDENSITY_4; /* 4uS */
 
 	ieee80211_ratectl_init(vap);
 	/* Complete setup. */