diff --git a/epan/dissectors/packet-l2tp.c b/epan/dissectors/packet-l2tp.c
index ede7cb8713..c32834ffbd 100644
--- a/epan/dissectors/packet-l2tp.c
+++ b/epan/dissectors/packet-l2tp.c
@@ -2933,12 +2933,18 @@ dissect_l2tp_udp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data
         return 0;
     }
 
+    /* RFCs 2661 and 3931 say that L2TPv2 and v3 use a TFTP-like method
+     * of each side choosing their own port and only using the L2TP port
+     * to establish the connection. In common practice, both parties use
+     * the assigned L2TP port the entire time, due to NAT, firewalls, etc.
+     * We support both methods by using conversations with no second port.
+     */
     conv = find_conversation(pinfo->num, &pinfo->src, &pinfo->dst, ENDPOINT_UDP,
                          pinfo->srcport, pinfo->destport, NO_PORT_B);
 
-    if (conv == NULL) {
-        conv = find_conversation(pinfo->num, &pinfo->src, &pinfo->dst, ENDPOINT_UDP,
-                             pinfo->srcport, pinfo->destport, 0);
+    if (conv == NULL || (conversation_get_dissector(conv, pinfo->num) != l2tp_udp_handle)) {
+        conv = find_conversation(pinfo->num, &pinfo->dst, &pinfo->src, ENDPOINT_UDP,
+                             pinfo->destport, pinfo->srcport, NO_PORT_B);
     }
 
     if ((conv == NULL) || (conversation_get_dissector(conv, pinfo->num) != l2tp_udp_handle)) {