diff --git a/config.c b/config.c
index c86ab01..cfe36b8 100644
--- a/config.c
+++ b/config.c
@@ -255,6 +255,12 @@ static enum parser_result parse_global_setting(const char *option,
 				dds->flags &= ~DDS_SLAVE_ONLY;
 		}
 
+	} else if (!strcmp(option, "gmCapable")) {
+		r = get_ranged_int(value, &val, 0, 1);
+		if (r != PARSED_OK)
+			return r;
+		cfg->dds.grand_master_capable = val;
+
 	} else if (!strcmp(option, "priority1")) {
 		r = get_ranged_uint(value, &uval, 0, UINT8_MAX);
 		if (r != PARSED_OK)
diff --git a/gPTP.cfg b/gPTP.cfg
index 3c0842c..249269e 100644
--- a/gPTP.cfg
+++ b/gPTP.cfg
@@ -3,7 +3,7 @@
 # Default Data Set
 #
 twoStepFlag		1
-slaveOnly		0
+gmCapable		1
 priority1		248
 priority2		248
 domainNumber		0
diff --git a/ptp4l.8 b/ptp4l.8
index c59b6e7..5cf642f 100644
--- a/ptp4l.8
+++ b/ptp4l.8
@@ -212,8 +212,15 @@ The default is 1 (enabled).
 .TP
 .B slaveOnly
 The local clock is a slave-only clock if enabled.
+This option is only for use with 1588 clocks and should not be enabled
+for 802.1AS clocks.
 The default is 0 (disabled).
 .TP
+.B gmCapable
+If this option is enabled, then the local clock is able to become grand master.
+This is only for use with 802.1AS clocks and has no effect on 1588 clocks.
+The default is 1 (enabled).
+.TP
 .B priority1
 The priority1 attribute of the local clock. It is used in the best master
 selection algorithm, lower values take precedence. Must be in the range 0 to
diff --git a/ptp4l.c b/ptp4l.c
index e86dd91..d5482ce 100644
--- a/ptp4l.c
+++ b/ptp4l.c
@@ -284,7 +284,14 @@ int main(int argc, char *argv[])
 	if (config && (c = config_read(config, &cfg_settings))) {
 		return c;
 	}
-	if (ds->flags & DDS_SLAVE_ONLY) {
+	if (!cfg_settings.dds.grand_master_capable &&
+	    ds->flags & DDS_SLAVE_ONLY) {
+		fprintf(stderr,
+			"Cannot mix 1588 slaveOnly with 802.1AS !gmCapable.\n");
+		return -1;
+	}
+	if (!cfg_settings.dds.grand_master_capable ||
+	    ds->flags & DDS_SLAVE_ONLY) {
 		ds->clockQuality.clockClass = 255;
 	}