bgpd: Display best path selection reason

As part of detailed bgp route detail, include the
reason why a route was selected as best path.

robot# show bgp ipv4 uni 223.255.254.0
BGP routing table entry for 223.255.254.0/24
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  annie(192.168.201.136)
  64539 15096 6939 7473 3758 55415
    192.168.201.136 from annie(192.168.201.136) (192.168.201.136)
      Origin IGP, valid, external, bestpath-from-AS 64539, best (First path received)
      Last update: Wed May 15 21:15:48 2019

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>

bgpd/bgp_route.c

@@ -7868,6 +7868,79 @@ static void route_vty_out_tx_ids(struct vty *vty,
 	}
 }
 
+static const char *bgp_path_selection_reason2str(
+	enum bgp_path_selection_reason reason)
+{
+	switch (reason) {
+	case bgp_path_selection_none:
+		return "Nothing to Select";
+		break;
+	case bgp_path_selection_first:
+		return "First path received";
+		break;
+	case bgp_path_selection_evpn_sticky_mac:
+		return "EVPN Sticky Mac";
+		break;
+	case bgp_path_selection_evpn_seq:
+		return "EVPN sequence number";
+		break;
+	case bgp_path_selection_evpn_lower_ip:
+		return "EVPN lower IP";
+		break;
+	case bgp_path_selection_weight:
+		return "Weight";
+		break;
+	case bgp_path_selection_local_pref:
+		return "Local Pref";
+		break;
+	case bgp_path_selection_local_route:
+		return "Local Route";
+		break;
+	case bgp_path_selection_confed_as_path:
+		return "Confederation based AS Path";
+		break;
+	case bgp_path_selection_as_path:
+		return "AS Path";
+		break;
+	case bgp_path_selection_origin:
+		return "Origin";
+		break;
+	case bgp_path_selection_med:
+		return "MED";
+		break;
+	case bgp_path_selection_peer:
+		return "Peer Type";
+		break;
+	case bgp_path_selection_confed:
+		return "Confed Peer Type";
+		break;
+	case bgp_path_selection_igp_metric:
+		return "IGP Metric";
+		break;
+	case bgp_path_selection_older:
+		return "Older Path";
+		break;
+	case bgp_path_selection_router_id:
+		return "Router ID";
+		break;
+	case bgp_path_selection_cluster_length:
+		return "Cluser length";
+		break;
+	case bgp_path_selection_stale:
+		return "Path Staleness";
+		break;
+	case bgp_path_selection_local_configured:
+		return "Locally configured route";
+		break;
+	case bgp_path_selection_neighbor_ip:
+		return "Neighbor IP";
+		break;
+	case bgp_path_selection_default:
+		return "Nothing left to compare";
+		break;
+	}
+}
+
 void route_vty_out_detail(struct vty *vty, struct bgp *bgp,
 			  struct bgp_node *bn, struct bgp_path_info *path,
 			  afi_t afi, safi_t safi, json_object *json_paths)
@@ -8463,8 +8536,14 @@ void route_vty_out_detail(struct vty *vty, struct bgp *bgp,
 						json_object_new_object();
 				json_object_boolean_true_add(json_bestpath,
 							     "overall");
-			} else
+				json_object_string_add(json_bestpath,
+						       "selectionReason",
+						       bgp_path_selection_reason2str(bn->reason));
+			} else {
 				vty_out(vty, ", best");
+				vty_out(vty, " (%s)",
+					bgp_path_selection_reason2str(bn->reason));
+			}
 		}
 
 		if (json_bestpath)