Negative `ip6_neighborgcthres' allows unlimited count of ND6 entries,
meanwhile positive value starts nd6_rtrequest() purging less recently
used ones.

ok?

Index: sys/netinet6/ip6_input.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_input.c,v
retrieving revision 1.294
diff -u -p -r1.294 ip6_input.c
--- sys/netinet6/ip6_input.c	31 Jul 2025 09:05:11 -0000	1.294
+++ sys/netinet6/ip6_input.c	2 Aug 2025 11:28:11 -0000
@@ -1459,7 +1459,7 @@ const struct sysctl_bounded_args ipv6ctl
 	{ IPV6CTL_MAXFRAGS, &ip6_maxfrags, 0, 1000 },
 	{ IPV6CTL_MFORWARDING, &ip6_mforwarding, 0, 1 },
 	{ IPV6CTL_MCAST_PMTU, &ip6_mcast_pmtu, 0, 1 },
-	{ IPV6CTL_NEIGHBORGCTHRESH, &ip6_neighborgcthresh, -1, 5 * 2048 },
+	{ IPV6CTL_NEIGHBORGCTHRESH, &ip6_neighborgcthresh, 0, 5 * 2048 },
 	{ IPV6CTL_MAXDYNROUTES, &ip6_maxdynroutes, -1, 5 * 4096 },
 };
 
Index: sys/netinet6/nd6.c
===================================================================
RCS file: /cvs/src/sys/netinet6/nd6.c,v
retrieving revision 1.293
diff -u -p -r1.293 nd6.c
--- sys/netinet6/nd6.c	26 Jul 2025 01:16:59 -0000	1.293
+++ sys/netinet6/nd6.c	2 Aug 2025 11:28:11 -0000
@@ -729,7 +729,6 @@ nd6_rtrequest(struct ifnet *ifp, int req
 	struct llinfo_nd6 *ln;
 	struct ifaddr *ifa;
 	struct in6_ifaddr *ifa6;
-	int ip6_neighborgcthresh_local;
 
 	if (ISSET(rt->rt_flags, RTF_GATEWAY|RTF_MULTICAST|RTF_MPLS))
 		return;
@@ -830,11 +829,7 @@ nd6_rtrequest(struct ifnet *ifp, int req
 		 * cause re-entering rtable related routines triggering
 		 * lock-order-reversal problems.
 		 */
-		ip6_neighborgcthresh_local =
-		    atomic_load_int(&ip6_neighborgcthresh);
-
-		if (ip6_neighborgcthresh_local >= 0 &&
-		    nd6_inuse >= ip6_neighborgcthresh_local) {
+		if (nd6_inuse >= atomic_load_int(&ip6_neighborgcthresh)) {
 			int i;
 
 			for (i = 0; i < 10; i++) {