bgpd: fix do re-send post-policy bgp update when not valid

When a BGP listener configured with BMP receives the first BGP
IPv6 update from a connected BGP IPv6 peer, the BMP collector
receives a withdraw post-policy message.

> {"peer_type": "route distinguisher instance", "policy": "post-policy",
> "ipv6": true, "peer_ip": "192:167::3", "peer_distinguisher": "444:1",
> "peer_asn": 65501, "peer_bgp_id": "192.168.1.3", "timestamp":
> "2024-10-29 11:44:47.111962", "bmp_log_type": "withdraw", "afi": 2,
> "safi": 1, "ip_prefix": "2001::1125/128", "seq": 22}
> {"peer_type": "route distinguisher instance", "policy": "pre-policy",
> "ipv6": true, "peer_ip": "192:167::3", "peer_distinguisher": "444:1",
> "peer_asn": 65501, "peer_bgp_id": "192.168.1.3", "timestamp":
> "2024-10-29 11:44:47.111963", "bmp_log_type": "update", "origin":
> "IGP", "as_path": "", "afi": 2, "safi": 1, "nxhp_ip": "192:167::3",
> "nxhp_link-local": "fe80::7063:d8ff:fedb:9e11", "ip_prefix": "2001::1125/128", "seq": 23}

Actually, the BGP update is not valid, and BMP considers it as a
withdraw message. The BGP upate is not valid, because the nexthop
reachability is unknown at the time of reception, and no other
BMP message is sent.

Fix this by re-sending a BMP post update message when nexthop
tracking becomes successfull. Generalise the re-sending of
messages when nexthop tracking changes.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>

bgpd/bgp_bmp.c

@@ -28,6 +28,7 @@
 #include "bgpd/bgp_table.h"
 #include "bgpd/bgpd.h"
 #include "bgpd/bgp_route.h"
+#include "bgpd/bgp_nht.h"
 #include "bgpd/bgp_attr.h"
 #include "bgpd/bgp_dump.h"
 #include "bgpd/bgp_errors.h"
@@ -1679,6 +1680,26 @@ static int bmp_process(struct bgp *bgp, afi_t afi, safi_t safi,
 	return 0;
 }
 
+static int bmp_nht_path_valid(struct bgp *bgp, struct bgp_path_info *path, bool valid)
+{
+	struct bgp_dest *dest = path->net;
+	struct bgp_table *table;
+
+	if (frrtrace_enabled(frr_bgp, bmp_nht_path_valid)) {
+		char pfxprint[PREFIX2STR_BUFFER];
+
+		prefix2str(&dest->rn->p, pfxprint, sizeof(pfxprint));
+		frrtrace(4, frr_bgp, bmp_nht_path_valid, bgp, pfxprint, path, valid);
+	}
+	if (bgp->peer_self == path->peer)
+		/* self declared networks or redistributed networks are not relevant for bmp */
+		return 0;
+
+	table = bgp_dest_table(dest);
+
+	return bmp_process(bgp, table->afi, table->safi, dest, path->peer, !valid);
+}
+
 static void bmp_stat_put_u32(struct stream *s, size_t *cnt, uint16_t type,
 		uint32_t value)
 {
@@ -3156,6 +3177,7 @@ static int bgp_bmp_module_init(void)
 	hook_register(peer_status_changed, bmp_peer_status_changed);
 	hook_register(peer_backward_transition, bmp_peer_backward);
 	hook_register(bgp_process, bmp_process);
+	hook_register(bgp_nht_path_update, bmp_nht_path_valid);
 	hook_register(bgp_inst_config_write, bmp_config_write);
 	hook_register(bgp_inst_delete, bmp_bgp_del);
 	hook_register(frr_late_init, bgp_bmp_init);

bgpd/bgp_nht.c

@@ -41,6 +41,9 @@ static void unregister_zebra_rnh(struct bgp_nexthop_cache *bnc);
 static int make_prefix(int afi, struct bgp_path_info *pi, struct prefix *p);
 static void bgp_nht_ifp_initial(struct event *thread);
 
+DEFINE_HOOK(bgp_nht_path_update, (struct bgp *bgp, struct bgp_path_info *pi, bool valid),
+	    (bgp, pi, valid));
+
 static int bgp_isvalid_nexthop(struct bgp_nexthop_cache *bnc)
 {
 	return (bgp_zebra_num_connects() == 0
@@ -1442,6 +1445,9 @@ void evaluate_paths(struct bgp_nexthop_cache *bnc)
 			}
 		}
 
+		if (path_valid != bnc_is_valid_nexthop)
+			hook_call(bgp_nht_path_update, bgp_path, path, bnc_is_valid_nexthop);
+
 		bgp_process(bgp_path, dest, path, afi, safi);
 	}
 

bgpd/bgp_nht.h

@@ -83,4 +83,9 @@ extern void bgp_nht_ifp_up(struct interface *ifp);
 extern void bgp_nht_ifp_down(struct interface *ifp);
 
 extern void bgp_nht_interface_events(struct peer *peer);
+
+/* called when a path becomes valid or invalid, because of nexthop tracking */
+DECLARE_HOOK(bgp_nht_path_update, (struct bgp *bgp, struct bgp_path_info *pi, bool valid),
+	     (bgp, pi, valid));
+
 #endif /* _BGP_NHT_H */

bgpd/bgp_trace.h

@@ -210,6 +210,24 @@ TRACEPOINT_EVENT(
 
 TRACEPOINT_LOGLEVEL(frr_bgp, bmp_process, TRACE_DEBUG)
 
+/*
+ * BMP is hooked for a nexthop tracking event
+ */
+TRACEPOINT_EVENT(
+	frr_bgp,
+	bmp_nht_path_valid,
+	TP_ARGS(struct bgp *, bgp, char *, pfx, struct bgp_path_info *,
+		path, bool, valid),
+	TP_FIELDS(
+		ctf_string(bgp, bgp->name_pretty)
+		ctf_string(prefix, pfx)
+		ctf_string(path, PEER_HOSTNAME(path->peer))
+		ctf_integer(bool, valid, valid)
+	)
+)
+
+TRACEPOINT_LOGLEVEL(frr_bgp, bmp_nht_path_valid, TRACE_DEBUG)
+
 /*
  * bgp_dest_lock/bgp_dest_unlock
  */