Index: sys/dev/sfxge/sfxge.c
===================================================================
--- sys/dev/sfxge/sfxge.c
+++ sys/dev/sfxge/sfxge.c
@@ -259,26 +259,34 @@
 			sfxge_mac_filter_set(sc);
 		break;
 	case SIOCSIFCAP:
+	{
+		int reqcap = ifr->ifr_reqcap;
+		int capchg_mask;
+
 		SFXGE_ADAPTER_LOCK(sc);
 
+		/* Capabilities to be changed in accordance with request */
+		capchg_mask = ifp->if_capenable ^ reqcap;
+
 		/*
 		 * The networking core already rejects attempts to
 		 * enable capabilities we don't have.  We still have
 		 * to reject attempts to disable capabilities that we
 		 * can't (yet) disable.
 		 */
-		if (~ifr->ifr_reqcap & SFXGE_CAP_FIXED) {
+		KASSERT(reqcap & ~ifp->if_capabilities,
+		    ("Unsupported capabilities requested"));
+		if (capchg_mask & SFXGE_CAP_FIXED) {
 			error = EINVAL;
 			SFXGE_ADAPTER_UNLOCK(sc);
 			break;
 		}
 
-		ifp->if_capenable = ifr->ifr_reqcap;
-		if (ifp->if_capenable & IFCAP_TXCSUM)
+		if (reqcap & IFCAP_TXCSUM)
 			ifp->if_hwassist |= (CSUM_IP | CSUM_TCP | CSUM_UDP);
 		else
 			ifp->if_hwassist &= ~(CSUM_IP | CSUM_TCP | CSUM_UDP);
-		if (ifp->if_capenable & IFCAP_TXCSUM_IPV6)
+		if (reqcap & IFCAP_TXCSUM_IPV6)
 			ifp->if_hwassist |= (CSUM_TCP_IPV6 | CSUM_UDP_IPV6);
 		else
 			ifp->if_hwassist &= ~(CSUM_TCP_IPV6 | CSUM_UDP_IPV6);
@@ -291,8 +299,11 @@
 		 * but both bits are set in IPv4 and IPv6 mbufs.
 		 */
 
+		ifp->if_capenable = reqcap;
+
 		SFXGE_ADAPTER_UNLOCK(sc);
 		break;
+	}
 	case SIOCSIFMEDIA:
 	case SIOCGIFMEDIA:
 		error = ifmedia_ioctl(ifp, ifr, &sc->media, command);