Index: head/share/man/man4/carp.4
===================================================================
--- head/share/man/man4/carp.4
+++ head/share/man/man4/carp.4
@@ -26,7 +26,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 7, 2018
+.Dd July 1, 2018
 .Dt CARP 4
 .Os
 .Sh NAME
@@ -104,6 +104,12 @@
 When enabled, a vhid in a backup state would preempt a master that
 is announcing itself with a lower advskew.
 Disabled by default.
+.It Va net.inet.carp.dscp
+DSCP value in carp packet.
+Valid Values are 0 to 63.
+A value of 4 is equivalent to the old standard of TOS LOW_DELAY.
+TOS values were deprecated and replaced by DSCP in 1998.
+The default value is 56 (CS7/Network Control).
 .It Va net.inet.carp.log
 Determines what events relating to
 .Nm
Index: head/sys/netinet/ip.h
===================================================================
--- head/sys/netinet/ip.h
+++ head/sys/netinet/ip.h
@@ -94,6 +94,11 @@
 #define	IPTOS_PREC_ROUTINE		IPTOS_DSCP_CS0
 
 /*
+ * Offset of Diffserv decimal value to convert it to tos value .
+ */
+#define	IPTOS_DSCP_OFFSET		2
+
+/*
  * Definitions for DiffServ Codepoints as per RFC2474 and RFC5865.
  */
 #define	IPTOS_DSCP_CS0		0x00
Index: head/sys/netinet/ip6.h
===================================================================
--- head/sys/netinet/ip6.h
+++ head/sys/netinet/ip6.h
@@ -104,6 +104,7 @@
 #define IPV6_FLOWLABEL_MASK	0xffff0f00	/* flow label (20 bits) */
 #endif /* LITTLE_ENDIAN */
 #endif
+#define IPV6_FLOWLABEL_LEN	20
 #if 1
 /* ECN bits proposed by Sally Floyd */
 #define IP6TOS_CE		0x01	/* congestion experienced */
Index: head/sys/netinet/ip_carp.c
===================================================================
--- head/sys/netinet/ip_carp.c
+++ head/sys/netinet/ip_carp.c
@@ -190,6 +190,10 @@
 static VNET_DEFINE(int, carp_allow) = 1;
 #define	V_carp_allow	VNET(carp_allow)
 
+/* Set DSCP in outgoing CARP packets. */
+static VNET_DEFINE(int, carp_dscp) = 56;
+#define	V_carp_dscp	VNET(carp_dscp)
+
 /* Preempt slower nodes. */
 static VNET_DEFINE(int, carp_preempt) = 0;
 #define	V_carp_preempt	VNET(carp_preempt)
@@ -211,12 +215,16 @@
 #define	V_carp_ifdown_adj	VNET(carp_ifdown_adj)
 
 static int carp_allow_sysctl(SYSCTL_HANDLER_ARGS);
+static int carp_dscp_sysctl(SYSCTL_HANDLER_ARGS);
 static int carp_demote_adj_sysctl(SYSCTL_HANDLER_ARGS);
 
 SYSCTL_NODE(_net_inet, IPPROTO_CARP,	carp,	CTLFLAG_RW, 0,	"CARP");
 SYSCTL_PROC(_net_inet_carp, OID_AUTO, allow,
     CTLFLAG_VNET | CTLTYPE_INT | CTLFLAG_RW, 0, 0, carp_allow_sysctl, "I",
     "Accept incoming CARP packets");
+SYSCTL_PROC(_net_inet_carp, OID_AUTO, dscp,
+    CTLFLAG_VNET | CTLTYPE_INT | CTLFLAG_RW, 0, 0, carp_dscp_sysctl, "I",
+    "DSCP value for carp packets");
 SYSCTL_INT(_net_inet_carp, OID_AUTO, preempt, CTLFLAG_VNET | CTLFLAG_RW,
     &VNET_NAME(carp_preempt), 0, "High-priority backup preemption mode");
 SYSCTL_INT(_net_inet_carp, OID_AUTO, log, CTLFLAG_VNET | CTLFLAG_RW,
@@ -933,7 +941,7 @@
 		ip = mtod(m, struct ip *);
 		ip->ip_v = IPVERSION;
 		ip->ip_hl = sizeof(*ip) >> 2;
-		ip->ip_tos = IPTOS_LOWDELAY;
+		ip->ip_tos = V_carp_dscp << IPTOS_DSCP_OFFSET;
 		ip->ip_len = htons(len);
 		ip->ip_off = htons(IP_DF);
 		ip->ip_ttl = CARP_DFLTTL;
@@ -983,6 +991,10 @@
 		ip6 = mtod(m, struct ip6_hdr *);
 		bzero(ip6, sizeof(*ip6));
 		ip6->ip6_vfc |= IPV6_VERSION;
+		/* Traffic class isn't defined in ip6 struct instead
+		 * it gets offset into flowid field */
+		ip6->ip6_flow |= htonl(V_carp_dscp << (IPV6_FLOWLABEL_LEN +
+		    IPTOS_DSCP_OFFSET));
 		ip6->ip6_hlim = CARP_DFLTTL;
 		ip6->ip6_nxt = IPPROTO_CARP;
 
@@ -2097,6 +2109,24 @@
 		}
 		mtx_unlock(&carp_mtx);
 	}
+
+	return (0);
+}
+
+static int
+carp_dscp_sysctl(SYSCTL_HANDLER_ARGS)
+{
+	int new, error;
+
+	new = V_carp_dscp;
+	error = sysctl_handle_int(oidp, &new, 0, req);
+	if (error || !req->newptr)
+		return (error);
+
+	if (new < 0 || new > 63)
+		return (EINVAL);
+
+	V_carp_dscp = new;
 
 	return (0);
 }