bgpd: add cli command to control explicit-null label usage

In BGP labeled unicast address-family, it is not possible to
send explicit-null label values with redistributed or network
declared prefixes.
A new CLI command is introduced:

  > [no] bgp labeled-unicast explicit-null

When used, the explicit-null value for IPv4 ('0' value) or
IPv6 ('2' value) will be used.
It is necessary to reconfigure the networks or the
redistribution in order to inherit this new behaviour.

Add the documentation.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>

bgpd/bgp_route.c

@@ -3070,7 +3070,9 @@ static void bgp_process_evpn_route_injection(struct bgp *bgp, afi_t afi,
  * the IMPLICIT_NULL label. This is pretty specialized: it's only called
  * in a path where we basically _know_ this is a BGP-LU route.
  */
-static bool bgp_lu_need_imp_null(const struct bgp_path_info *new_select)
+static bool bgp_lu_need_null_label(struct bgp *bgp,
+				   const struct bgp_path_info *new_select,
+				   afi_t afi, mpls_label_t *label)
 {
 	/* Certain types get imp null; so do paths where the nexthop is
 	 * not labeled.
@@ -3079,12 +3081,20 @@ static bool bgp_lu_need_imp_null(const struct bgp_path_info *new_select)
 	    || new_select->sub_type == BGP_ROUTE_AGGREGATE
 	    || new_select->sub_type == BGP_ROUTE_REDISTRIBUTE)
 		return true;
-	else if (new_select->extra == NULL ||
-		 !bgp_is_valid_label(&new_select->extra->label[0]))
-		/* TODO -- should be configurable? */
-		return true;
-	else
+	else if (new_select->extra &&
+		 bgp_is_valid_label(&new_select->extra->label[0]))
 		return false;
+	if (label == NULL)
+		return true;
+	if (!!CHECK_FLAG(bgp->flags, BGP_FLAG_LU_EXPLICIT_NULL))
+		/* Disable PHP : explicit-null */
+		*label = afi == AFI_IP ? MPLS_LABEL_IPV4_EXPLICIT_NULL
+				       : MPLS_LABEL_IPV6_EXPLICIT_NULL;
+	else
+		/* Enforced PHP popping: implicit-null */
+		*label = MPLS_LABEL_IMPLICIT_NULL;
+
+	return true;
 }
 
 /*
@@ -3113,6 +3123,7 @@ static void bgp_process_main_one(struct bgp *bgp, struct bgp_dest *dest,
 	struct bgp_path_info *old_select;
 	struct bgp_path_info_pair old_and_new;
 	int debug = 0;
+	mpls_label_t mpls_label_null;
 
 	if (CHECK_FLAG(bgp->flags, BGP_FLAG_DELETE_IN_PROGRESS)) {
 		if (dest)
@@ -3167,7 +3178,7 @@ static void bgp_process_main_one(struct bgp *bgp, struct bgp_dest *dest,
 	 * Right now, since we only deal with per-prefix labels, it is not
 	 * necessary to do this upon changes to best path. Exceptions:
 	 * - label index has changed -> recalculate resulting label
-	 * - path_info sub_type changed -> switch to/from implicit-null
+	 * - path_info sub_type changed -> switch to/from null label value
 	 * - no valid label (due to removed static label binding) -> get new one
 	 */
 	if (bgp->allocate_mpls_labels[afi][safi]) {
@@ -3176,11 +3187,12 @@ static void bgp_process_main_one(struct bgp *bgp, struct bgp_dest *dest,
 			    || bgp_label_index_differs(new_select, old_select)
 			    || new_select->sub_type != old_select->sub_type
 			    || !bgp_is_valid_label(&dest->local_label)) {
-				/* Enforced penultimate hop popping:
-				 * implicit-null for local routes, aggregate
-				 * and redistributed routes
+				/* control label imposition for local routes,
+				 * aggregate and redistributed routes
 				 */
-				if (bgp_lu_need_imp_null(new_select)) {
+				mpls_label_null = MPLS_LABEL_IMPLICIT_NULL;
+				if (bgp_lu_need_null_label(bgp, new_select, afi,
+							   &mpls_label_null)) {
 					if (CHECK_FLAG(
 						    dest->flags,
 						    BGP_NODE_REGISTERED_FOR_LABEL)
@@ -3189,8 +3201,7 @@ static void bgp_process_main_one(struct bgp *bgp, struct bgp_dest *dest,
 						    BGP_NODE_LABEL_REQUESTED))
 						bgp_unregister_for_label(dest);
 					dest->local_label = mpls_lse_encode(
-						MPLS_LABEL_IMPLICIT_NULL, 0, 0,
-						1);
+						mpls_label_null, 0, 0, 1);
 					bgp_set_valid_label(&dest->local_label);
 				} else
 					bgp_register_for_label(dest,

bgpd/bgp_vty.c

@@ -2820,6 +2820,21 @@ DEFUN(no_bgp_ebgp_requires_policy, no_bgp_ebgp_requires_policy_cmd,
 	return CMD_SUCCESS;
 }
 
+DEFPY(bgp_lu_uses_explicit_null, bgp_lu_uses_explicit_null_cmd,
+      "[no] bgp labeled-unicast explicit-null",
+      NO_STR BGP_STR
+      "BGP Labeled-unicast options\n"
+      "Use explicit-null label values for local prefixes\n")
+{
+	VTY_DECLVAR_CONTEXT(bgp, bgp);
+
+	if (no)
+		UNSET_FLAG(bgp->flags, BGP_FLAG_LU_EXPLICIT_NULL);
+	else
+		SET_FLAG(bgp->flags, BGP_FLAG_LU_EXPLICIT_NULL);
+	return CMD_SUCCESS;
+}
+
 DEFUN(bgp_suppress_duplicates, bgp_suppress_duplicates_cmd,
       "bgp suppress-duplicates",
       BGP_STR
@@ -18235,6 +18250,9 @@ int bgp_config_write(struct vty *vty)
 					? ""
 					: "no ");
 
+		if (!!CHECK_FLAG(bgp->flags, BGP_FLAG_LU_EXPLICIT_NULL))
+			vty_out(vty, " bgp labeled-unicast explicit-null\n");
+
 		/* draft-ietf-idr-deprecate-as-set-confed-set */
 		if (bgp->reject_as_sets)
 			vty_out(vty, " bgp reject-as-sets\n");
@@ -19154,6 +19172,9 @@ void bgp_vty_init(void)
 	install_element(BGP_NODE, &bgp_ebgp_requires_policy_cmd);
 	install_element(BGP_NODE, &no_bgp_ebgp_requires_policy_cmd);
 
+	/* bgp labeled-unicast explicit-null */
+	install_element(BGP_NODE, &bgp_lu_uses_explicit_null_cmd);
+
 	/* bgp suppress-duplicates */
 	install_element(BGP_NODE, &bgp_suppress_duplicates_cmd);
 	install_element(BGP_NODE, &no_bgp_suppress_duplicates_cmd);

bgpd/bgpd.h

@@ -500,6 +500,8 @@ struct bgp {
 #define BGP_FLAG_HARD_ADMIN_RESET (1ULL << 31)
 /* Evaluate the AIGP attribute during the best path selection process */
 #define BGP_FLAG_COMPARE_AIGP (1ULL << 32)
+/* For BGP-LU, force local prefixes to use explicit-null label */
+#define BGP_FLAG_LU_EXPLICIT_NULL (1ULL << 33)
 
 	/* BGP default address-families.
 	 * New peers inherit enabled afi/safis from bgp instance.

doc/user/bgp.rst

@@ -2767,6 +2767,17 @@ happened automatically if local-role is set.
    value of his role (by setting local-role on his side). Otherwise, a Role
    Mismatch Notification will be sent.
 
+Labeled unicast
+---------------
+
+*bgpd* supports labeled information, as per :rfc:`3107`.
+
+.. clicmd:: bgp labeled-unicast explicit-null
+
+By default, locally advertised prefixes use the `implicit-null` label to
+encode in the outgoing NLRI. The following command uses the `explicit-null`
+label value for all the BGP instances.
+
 .. _bgp-l3vpn-vrfs:
 
 L3VPN VRFs