lacp_fallback defines the behavior of a LACP bonding interface
when no slaves are in collecting(/distributing) state while at least
'min_links' slaves have carrier.

In the default (legacy) mode, the bonding master remains up and a
single slave is selected for TX/RX, while traffic received on other
slaves is dropped. This preserves the existing behavior.

In strict mode, the bonding master reports carrier down in this
situation.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
---
 include/uapi/linux/if_link.h |  1 +
 ip/iplink_bond.c             | 25 +++++++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 2037afbc..1588d520 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -1537,6 +1537,7 @@ enum {
 	IFLA_BOND_NS_IP6_TARGET,
 	IFLA_BOND_COUPLED_CONTROL,
 	IFLA_BOND_BROADCAST_NEIGH,
+	IFLA_BOND_LACP_FALLBACK,
 	__IFLA_BOND_MAX,
 };
 
diff --git a/ip/iplink_bond.c b/ip/iplink_bond.c
index 714fe7bd..f33252e8 100644
--- a/ip/iplink_bond.c
+++ b/ip/iplink_bond.c
@@ -87,6 +87,12 @@ static const char *lacp_rate_tbl[] = {
 	NULL,
 };
 
+static const char *lacp_fallback_tbl[] = {
+	"legacy",
+	"strict",
+	NULL,
+};
+
 static const char *ad_select_tbl[] = {
 	"stable",
 	"bandwidth",
@@ -155,6 +161,7 @@ static void print_explain(FILE *f)
 		"                [ ad_user_port_key PORTKEY ]\n"
 		"                [ ad_actor_sys_prio SYSPRIO ]\n"
 		"                [ ad_actor_system LLADDR ]\n"
+		"                [ lacp_fallback LACP_FALLBACK ]\n"
 		"                [ arp_missed_max MISSED_MAX ]\n"
 		"\n"
 		"BONDMODE := balance-rr|active-backup|balance-xor|broadcast|802.3ad|balance-tlb|balance-alb\n"
@@ -168,6 +175,7 @@ static void print_explain(FILE *f)
 		"AD_SELECT := stable|bandwidth|count\n"
 		"COUPLED_CONTROL := off|on\n"
 		"BROADCAST_NEIGHBOR := off|on\n"
+		"LACP_FALLBACK := legacy|strict\n"
 	);
 }
 
@@ -188,6 +196,7 @@ static int bond_parse_opt(struct link_util *lu, int argc, char **argv,
 	__u32 packets_per_slave;
 	__u8 missed_max;
 	__u8 broadcast_neighbor;
+	__u8 lacp_fallback;
 	unsigned int ifindex;
 	int ret;
 
@@ -417,6 +426,13 @@ static int bond_parse_opt(struct link_util *lu, int argc, char **argv,
 				return -1;
 			addattr_l(n, 1024, IFLA_BOND_AD_ACTOR_SYSTEM,
 				  abuf, len);
+		} else if (matches(*argv, "lacp_fallback") == 0) {
+			NEXT_ARG();
+			if (get_index(lacp_fallback_tbl, *argv) < 0)
+				invarg("invalid lacp_rate", *argv);
+
+			lacp_fallback = get_index(lacp_fallback_tbl, *argv);
+			addattr8(n, 1024, IFLA_BOND_LACP_FALLBACK, lacp_fallback);
 		} else if (matches(*argv, "tlb_dynamic_lb") == 0) {
 			NEXT_ARG();
 			if (get_u8(&tlb_dynamic_lb, *argv, 0)) {
@@ -642,6 +658,15 @@ static void bond_print_opt(struct link_util *lu, FILE *f, struct rtattr *tb[])
 			   "all_slaves_active %u ",
 			   rta_getattr_u8(tb[IFLA_BOND_ALL_SLAVES_ACTIVE]));
 
+	if (tb[IFLA_BOND_LACP_FALLBACK]) {
+		const char *lacp_fallback = get_name(lacp_fallback_tbl,
+						 rta_getattr_u8(tb[IFLA_BOND_LACP_FALLBACK]));
+		print_string(PRINT_ANY,
+			     "lacp_fallback",
+			     "lacp_fallback %s ",
+			     lacp_fallback);
+	}
+
 	if (tb[IFLA_BOND_MIN_LINKS])
 		print_uint(PRINT_ANY,
 			   "min_links",
-- 
2.39.2