bgpd: Send BGP best path reason to Zebra

```
exit1-debian-9# show ip route 172.16.16.1/32
Routing entry for 172.16.16.1/32
  Known via "bgp", distance 20, metric 0, best
  Last update 00:00:28 ago
  * 192.168.0.2, via eth1, weight 1
    AS-Path          : 65003
    Communities      : first 65001:2 65001:3
    Large-Communities: 65001:1:1 65001:1:2 65001:1:3
    Selection reason : First path received
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

5 files changed, 22 insertions, 2 deletions

diff --git a/bgpd/bgp_route.c b/bgpd/bgp_route.c
index 0eb3cc61e..f2a3e07ae 100644
--- a/bgpd/bgp_route.c
+++ b/bgpd/bgp_route.c
@@ -8415,8 +8415,7 @@ enum bgp_display_type {
 	normal_list,
 };
 
-static const char *
-bgp_path_selection_reason2str(enum bgp_path_selection_reason reason)
+const char *bgp_path_selection_reason2str(enum bgp_path_selection_reason reason)
 {
 	switch (reason) {
 	case bgp_path_selection_none:
diff --git a/bgpd/bgp_route.h b/bgpd/bgp_route.h
index 37bf675b6..46802d0d1 100644
--- a/bgpd/bgp_route.h
+++ b/bgpd/bgp_route.h
@@ -90,6 +90,9 @@ enum bgp_show_adj_route_type {
 /* Maximum number of sids we can process or send with a prefix. */
 #define BGP_MAX_SIDS 6
 
+/* Maximum buffer length for storing BGP best path selection reason */
+#define BGP_MAX_SELECTION_REASON_STR_BUF 32
+
 /* Error codes for handling NLRI */
 #define BGP_NLRI_PARSE_OK 0
 #define BGP_NLRI_PARSE_ERROR_PREFIX_OVERFLOW -1
@@ -803,4 +806,6 @@ extern void bgp_aggregate_toggle_suppressed(struct bgp_aggregate *aggregate,
 					    const struct prefix *p, afi_t afi,
 					    safi_t safi, bool suppress);
 extern void subgroup_announce_reset_nhop(uint8_t family, struct attr *attr);
+const char *
+bgp_path_selection_reason2str(enum bgp_path_selection_reason reason);
 #endif /* _QUAGGA_BGP_ROUTE_H */
diff --git a/bgpd/bgp_zebra.c b/bgpd/bgp_zebra.c
index 0249d53f0..a11a350fd 100644
--- a/bgpd/bgp_zebra.c
+++ b/bgpd/bgp_zebra.c
@@ -1489,6 +1489,8 @@ void bgp_zebra_announce(struct bgp_dest *dest, const struct prefix *p,
 
 	if (is_add && CHECK_FLAG(bm->flags, BM_FLAG_SEND_EXTRA_DATA_TO_ZEBRA)) {
 		struct bgp_zebra_opaque bzo = {};
+		const char *reason =
+			bgp_path_selection_reason2str(dest->reason);
 
 		strlcpy(bzo.aspath, info->attr->aspath->str,
 			sizeof(bzo.aspath));
@@ -1502,6 +1504,9 @@ void bgp_zebra_announce(struct bgp_dest *dest, const struct prefix *p,
 			strlcpy(bzo.lcommunity, info->attr->lcommunity->str,
 				sizeof(bzo.lcommunity));
 
+		strlcpy(bzo.selection_reason, reason,
+			sizeof(bzo.selection_reason));
+
 		SET_FLAG(api.message, ZAPI_MESSAGE_OPAQUE);
 		api.opaque.length = MIN(sizeof(struct bgp_zebra_opaque),
 					ZAPI_MESSAGE_OPAQUE_LENGTH);
diff --git a/lib/route_opaque.h b/lib/route_opaque.h
index 7c4e9a16e..c5e7d6a32 100644
--- a/lib/route_opaque.h
+++ b/lib/route_opaque.h
@@ -36,6 +36,12 @@ struct bgp_zebra_opaque {
 
 	/* Show at least 10 large-communities AA:BB:CC */
 	char lcommunity[LCOMMUNITY_SIZE * 30];
+
+	/* 32 bytes seems enough because of
+	 * bgp_path_selection_confed_as_path which is
+	 * `Confederation based AS Path`.
+	 */
+	char selection_reason[BGP_MAX_SELECTION_REASON_STR_BUF];
 };
 
 static_assert(sizeof(struct bgp_zebra_opaque) <= ZAPI_MESSAGE_OPAQUE_LENGTH,
diff --git a/zebra/zebra_vty.c b/zebra/zebra_vty.c
index 06528d3db..3d101011e 100644
--- a/zebra/zebra_vty.c
+++ b/zebra/zebra_vty.c
@@ -449,6 +449,8 @@ static void zebra_show_ip_route_opaque(struct vty *vty, struct route_entry *re,
 					       bzo.community);
 			json_object_string_add(json, "largeCommunities",
 					       bzo.lcommunity);
+			json_object_string_add(json, "selectionReason",
+					       bzo.selection_reason);
 		} else {
 			vty_out(vty, "    AS-Path          : %s\n", bzo.aspath);
 
@@ -459,6 +461,9 @@ static void zebra_show_ip_route_opaque(struct vty *vty, struct route_entry *re,
 			if (bzo.lcommunity[0] != '\0')
 				vty_out(vty, "    Large-Communities: %s\n",
 					bzo.lcommunity);
+
+			vty_out(vty, "    Selection reason : %s\n",
+				bzo.selection_reason);
 		}
 	}
 	default: