diff --git a/sys/netinet6/icmp6.c b/sys/netinet6/icmp6.c
--- a/sys/netinet6/icmp6.c
+++ b/sys/netinet6/icmp6.c
@@ -1111,6 +1111,7 @@
 	struct mbuf *m = ip6cp->ip6c_m;	/* will be necessary for scope issue */
 	u_int mtu = ntohl(icmp6->icmp6_mtu);
 	struct in_conninfo inc;
+	uint32_t max_mtu;
 
 #if 0
 	/*
@@ -1151,7 +1152,11 @@
 	if (in6_setscope(&inc.inc6_faddr, m->m_pkthdr.rcvif, NULL))
 		return;
 
-	if (mtu < tcp_maxmtu6(&inc, NULL)) {
+	max_mtu = tcp_hc_getmtu(&inc);
+	if (max_mtu == 0)
+		max_mtu = tcp_maxmtu6(&inc, NULL);
+
+	if (mtu < max_mtu) {
 		tcp_hc_updatemtu(&inc, mtu);
 		ICMP6STAT_INC(icp6s_pmtuchg);
 	}