Index: src/util.h
===================================================================
--- src/util.h	(revision 2480)
+++ src/util.h	(working copy)
@@ -98,6 +98,7 @@ extern u_int32_t GenerateMagic(void);
 extern int GetAnyIpAddress(struct u_addr *ipaddr, const char *ifname);
 extern int GetEther(struct u_addr *addr, struct sockaddr_dl *hwaddr);
 extern int GetPeerEther(struct u_addr *addr, struct sockaddr_dl *hwaddr);
+extern uint16_t GetSystemIfaceMTU(const char *ifname);
 extern void ppp_util_ascify(char *buf, size_t max, const char *bytes, size_t len);
 extern int IfaceSetFlag(const char *ifname, unsigned value);
 
Index: src/util.c
===================================================================
--- src/util.c	(revision 2480)
+++ src/util.c	(working copy)
@@ -1597,3 +1597,25 @@ ssize_t GetDataAddrs(int sock, void *dbuf, size_t dbuf
 
 	return (size);
 }
+
+uint16_t GetSystemIfaceMTU(const char *ifname)
+{
+	struct ifreq ifr;
+	static int sock = -1;
+
+	if (sock == -1 && (sock = socket(PF_INET, SOCK_DGRAM, 0)) == -1) {
+		Perror("[%s] %s: Socket creation error",
+			ifname, __FUNCTION__);
+		return (0);
+	}
+
+	memset(&ifr, 0, sizeof(ifr));
+	strlcpy(ifr.ifr_name, ifname, sizeof(ifr.ifr_name));
+
+	if (ioctl(sock, SIOCGIFMTU, (caddr_t)&ifr) == -1) {
+		Perror("[%s] %s: SIOCGIFMTU failed", ifname, __FUNCTION__);
+		return (0);
+	}
+	/* Let _exit() close sock */
+	return (ifr.ifr_mtu);
+}
Index: src/pppoe.c
===================================================================
--- src/pppoe.c	(revision 2480)
+++ src/pppoe.c	(working copy)
@@ -1712,6 +1712,7 @@ PppoeSetCommand(Context ctx, int ac, const char *const
 	unsigned i;
 #ifdef NGM_PPPOE_SETMAXP_COOKIE
 	int ap;
+	uint16_t mtu;
 #endif
 	switch ((intptr_t)arg) {
 	case SET_IFACE:
@@ -1732,6 +1733,20 @@ PppoeSetCommand(Context ctx, int ac, const char *const
 				}
 			}
 			strlcpy(pi->hook, hookname, sizeof(pi->hook));
+
+#ifdef NGM_PPPOE_SETMAXP_COOKIE
+			if (pi->max_payload > 0) {
+				mtu = GetSystemIfaceMTU(pi->iface);
+				if (mtu == 0)
+					mtu = ETHER_MAX_LEN;
+				if (pi->max_payload > mtu - 8) {
+					pi->max_payload = mtu - 8;
+					Perror("[%s] PPPoE: PPP-Max-Payload"
+					       " value reduced to %hu",
+						pi->iface, pi->max_payload);
+				}
+			}
+#endif
 			break;
 		default:
 			return(-1);
@@ -1762,8 +1777,18 @@ PppoeSetCommand(Context ctx, int ac, const char *const
 		if (ac != 1)
 			return(-1);
 		ap = atoi(av[0]);
-		if (ap < PPPOE_MRU || ap > ETHER_MAX_LEN - 8)
-			Error("PPP-Max-Payload value \"%s\"", av[0]);
+		if (pi->iface[0] == '\0') {
+			if (ap < PPPOE_MRU)	/* postpone check for MTU */
+			    Error("PPP-Max-Payload value \"%s\" less than %d",
+				av[0], PPPOE_MRU);
+		} else {
+			mtu = GetSystemIfaceMTU(pi->iface);
+			if (mtu == 0)
+				mtu = ETHER_MAX_LEN;
+			if (ap < PPPOE_MRU || ap > mtu - 8)
+			    Error("PPP-Max-Payload value \"%s\" not in a range of %d..%hu",
+				av[0], PPPOE_MRU, mtu);
+		}
 		pi->max_payload = ap;
 		break;
 #endif