diff --git a/sys/net/ethernet.h b/sys/net/ethernet.h
--- a/sys/net/ethernet.h
+++ b/sys/net/ethernet.h
@@ -446,7 +446,7 @@
 	    void *, u_int);
 struct mbuf  *ether_vlanencap_proto(struct mbuf *, uint16_t, uint16_t);
 bool	ether_8021q_frame(struct mbuf **mp, struct ifnet *ife,
-		struct ifnet *p, struct ether_8021q_tag *);
+		struct ifnet *p, const struct ether_8021q_tag *);
 void	ether_gen_addr(struct ifnet *ifp, struct ether_addr *hwaddr);
 
 static __inline struct mbuf *ether_vlanencap(struct mbuf *m, uint16_t tag)
diff --git a/sys/net/if_ethersubr.c b/sys/net/if_ethersubr.c
--- a/sys/net/if_ethersubr.c
+++ b/sys/net/if_ethersubr.c
@@ -1400,11 +1400,12 @@
 
 bool
 ether_8021q_frame(struct mbuf **mp, struct ifnet *ife, struct ifnet *p,
-    struct ether_8021q_tag *qtag)
+    const struct ether_8021q_tag *qtag)
 {
 	struct m_tag *mtag;
 	int n;
 	uint16_t tag;
+	uint8_t pcp = qtag->pcp;
 	static const char pad[8];	/* just zeros */
 
 	/*
@@ -1437,7 +1438,7 @@
 	 * If PCP is set in mbuf, use it
 	 */
 	if ((*mp)->m_flags & M_VLANTAG) {
-		qtag->pcp = EVL_PRIOFTAG((*mp)->m_pkthdr.ether_vtag);
+		pcp = EVL_PRIOFTAG((*mp)->m_pkthdr.ether_vtag);
 	}
 
 	/*
@@ -1451,7 +1452,7 @@
 	    MTAG_8021Q_PCP_OUT, NULL)) != NULL)
 		tag = EVL_MAKETAG(qtag->vid, *(uint8_t *)(mtag + 1), 0);
 	else
-		tag = EVL_MAKETAG(qtag->vid, qtag->pcp, 0);
+		tag = EVL_MAKETAG(qtag->vid, pcp, 0);
 	if ((p->if_capenable & IFCAP_VLAN_HWTAGGING) &&
 	    (qtag->proto == ETHERTYPE_VLAN)) {
 		(*mp)->m_pkthdr.ether_vtag = tag;