bgpd: Implement BGP dual-as feature

This is helpful for migrations, etc.

The neighbor is configured with:

```
router bgp 65000
 neighbor X local-as 65001 no-prepend replace-as dual-as
```

Neighbor X can use either 65000, or 65001 to peer with.

Closes: https://github.com/FRRouting/frr/issues/13928

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

5 files changed, 59 insertions, 20 deletions

diff --git a/bgpd/bgp_packet.c b/bgpd/bgp_packet.c
index fa03f1d21d..62be7ffbf7 100644
--- a/bgpd/bgp_packet.c
+++ b/bgpd/bgp_packet.c
@@ -2702,6 +2702,19 @@ static int bgp_notify_receive(struct peer_connection *connection,
 	    inner.subcode == BGP_NOTIFY_OPEN_UNSUP_PARAM)
 		UNSET_FLAG(peer->sflags, PEER_STATUS_CAPABILITY_OPEN);
 
+	/* Resend the next OPEN message with a global AS number if we received
+	 * a `Bad Peer AS` notification. This is only valid if `dual-as` is
+	 * configured.
+	 */
+	if (inner.code == BGP_NOTIFY_OPEN_ERR &&
+	    inner.subcode == BGP_NOTIFY_OPEN_BAD_PEER_AS &&
+	    CHECK_FLAG(peer->flags, PEER_FLAG_DUAL_AS)) {
+		if (peer->change_local_as != peer->bgp->as)
+			peer->change_local_as = peer->bgp->as;
+		else
+			peer->change_local_as = peer->local_as;
+	}
+
 	/* If Graceful-Restart N-bit (Notification) is exchanged,
 	 * and it's not a Hard Reset, let's retain the routes.
 	 */
diff --git a/bgpd/bgp_updgrp.c b/bgpd/bgp_updgrp.c
index 115bc35cdc..90c43b938f 100644
--- a/bgpd/bgp_updgrp.c
+++ b/bgpd/bgp_updgrp.c
@@ -783,8 +783,11 @@ static int update_group_show_walkcb(struct update_group *updgrp, void *arg)
 				json_updgrp, "replaceLocalAs",
 				CHECK_FLAG(updgrp->conf->flags,
 					   PEER_FLAG_LOCAL_AS_REPLACE_AS));
+			json_object_boolean_add(json_updgrp, "dualAs",
+						CHECK_FLAG(updgrp->conf->flags,
+							   PEER_FLAG_DUAL_AS));
 		} else {
-			vty_out(vty, "  Local AS %u%s%s\n",
+			vty_out(vty, "  Local AS %u%s%s%s\n",
 				updgrp->conf->change_local_as,
 				CHECK_FLAG(updgrp->conf->flags,
 					   PEER_FLAG_LOCAL_AS_NO_PREPEND)
@@ -793,6 +796,10 @@ static int update_group_show_walkcb(struct update_group *updgrp, void *arg)
 				CHECK_FLAG(updgrp->conf->flags,
 					   PEER_FLAG_LOCAL_AS_REPLACE_AS)
 					? " replace-as"
+					: "",
+				CHECK_FLAG(updgrp->conf->flags,
+					   PEER_FLAG_DUAL_AS)
+					? " dual-as"
 					: "");
 		}
 	}
diff --git a/bgpd/bgp_vty.c b/bgpd/bgp_vty.c
index f669564bb8..cf4269eb66 100644
--- a/bgpd/bgp_vty.c
+++ b/bgpd/bgp_vty.c
@@ -5451,7 +5451,7 @@ DEFUN (neighbor_local_as,
 		return CMD_WARNING_CONFIG_FAILED;
 	}
 
-	ret = peer_local_as_set(peer, as, 0, 0, argv[idx_number]->arg);
+	ret = peer_local_as_set(peer, as, 0, 0, 0, argv[idx_number]->arg);
 	return bgp_vty_return(vty, ret);
 }
 
@@ -5480,19 +5480,20 @@ DEFUN (neighbor_local_as_no_prepend,
 		return CMD_WARNING_CONFIG_FAILED;
 	}
 
-	ret = peer_local_as_set(peer, as, 1, 0, argv[idx_number]->arg);
+	ret = peer_local_as_set(peer, as, 1, 0, 0, argv[idx_number]->arg);
 	return bgp_vty_return(vty, ret);
 }
 
-DEFUN (neighbor_local_as_no_prepend_replace_as,
+DEFPY (neighbor_local_as_no_prepend_replace_as,
        neighbor_local_as_no_prepend_replace_as_cmd,
-       "neighbor <A.B.C.D|X:X::X:X|WORD> local-as ASNUM no-prepend replace-as",
+       "neighbor <A.B.C.D|X:X::X:X|WORD> local-as ASNUM no-prepend replace-as [dual-as$dual_as]",
        NEIGHBOR_STR
        NEIGHBOR_ADDR_STR2
        "Specify a local-as number\n"
        "AS number expressed in dotted or plain format used as local AS\n"
        "Do not prepend local-as to updates from ebgp peers\n"
-       "Do not prepend local-as to updates from ibgp peers\n")
+       "Do not prepend local-as to updates from ibgp peers\n"
+       "Allow peering with a global AS number or local-as number\n")
 {
 	int idx_peer = 1;
 	int idx_number = 3;
@@ -5510,20 +5511,21 @@ DEFUN (neighbor_local_as_no_prepend_replace_as,
 		return CMD_WARNING_CONFIG_FAILED;
 	}
 
-	ret = peer_local_as_set(peer, as, 1, 1, argv[idx_number]->arg);
+	ret = peer_local_as_set(peer, as, 1, 1, dual_as, argv[idx_number]->arg);
 	return bgp_vty_return(vty, ret);
 }
 
 DEFUN (no_neighbor_local_as,
        no_neighbor_local_as_cmd,
-       "no neighbor <A.B.C.D|X:X::X:X|WORD> local-as [ASNUM [no-prepend [replace-as]]]",
+       "no neighbor <A.B.C.D|X:X::X:X|WORD> local-as [ASNUM [no-prepend [replace-as] [dual-as]]]",
        NO_STR
        NEIGHBOR_STR
        NEIGHBOR_ADDR_STR2
        "Specify a local-as number\n"
        "AS number expressed in dotted or plain format used as local AS\n"
        "Do not prepend local-as to updates from ebgp peers\n"
-       "Do not prepend local-as to updates from ibgp peers\n")
+       "Do not prepend local-as to updates from ibgp peers\n"
+       "Allow peering with a global AS number or local-as number\n")
 {
 	int idx_peer = 2;
 	struct peer *peer;
@@ -14052,6 +14054,10 @@ static void bgp_show_peer(struct vty *vty, struct peer *p, bool use_json,
 		if (CHECK_FLAG(p->flags, PEER_FLAG_LOCAL_AS_REPLACE_AS))
 			json_object_boolean_true_add(json_neigh,
 						     "localAsReplaceAs");
+
+		json_object_boolean_add(json_neigh, "localAsReplaceAsDualAs",
+					!!CHECK_FLAG(p->flags,
+						     PEER_FLAG_DUAL_AS));
 	} else {
 		if (p->as_type == AS_SPECIFIED ||
 		    CHECK_FLAG(p->as_type, AS_AUTO) ||
@@ -14066,13 +14072,15 @@ static void bgp_show_peer(struct vty *vty, struct peer *p, bool use_json,
 		vty_out(vty, ASN_FORMAT(bgp->asnotation),
 			p->change_local_as ? &p->change_local_as
 					   : &p->local_as);
-		vty_out(vty, "%s%s, ",
+		vty_out(vty, "%s%s%s, ",
 			CHECK_FLAG(p->flags, PEER_FLAG_LOCAL_AS_NO_PREPEND)
 				? " no-prepend"
 				: "",
 			CHECK_FLAG(p->flags, PEER_FLAG_LOCAL_AS_REPLACE_AS)
 				? " replace-as"
-				: "");
+				: "",
+			CHECK_FLAG(p->flags, PEER_FLAG_DUAL_AS) ? " dual-as"
+								: "");
 	}
 	/* peer type internal or confed-internal */
 	if ((p->as == p->local_as) || (CHECK_FLAG(p->as_type, AS_INTERNAL))) {
@@ -18664,6 +18672,8 @@ static void bgp_config_write_peer_global(struct vty *vty, struct bgp *bgp,
 			vty_out(vty, " no-prepend");
 		if (peergroup_flag_check(peer, PEER_FLAG_LOCAL_AS_REPLACE_AS))
 			vty_out(vty, " replace-as");
+		if (peergroup_flag_check(peer, PEER_FLAG_DUAL_AS))
+			vty_out(vty, " dual-as");
 		vty_out(vty, "\n");
 	}
 
diff --git a/bgpd/bgpd.c b/bgpd/bgpd.c
index 9d36ed9008..1108b82f4e 100644
--- a/bgpd/bgpd.c
+++ b/bgpd/bgpd.c
@@ -4696,6 +4696,7 @@ static const struct peer_flag_action peer_flag_action_list[] = {
 	{PEER_FLAG_LOCAL_AS, 0, peer_change_reset},
 	{PEER_FLAG_LOCAL_AS_NO_PREPEND, 0, peer_change_reset},
 	{PEER_FLAG_LOCAL_AS_REPLACE_AS, 0, peer_change_reset},
+	{PEER_FLAG_DUAL_AS, 0, peer_change_reset},
 	{PEER_FLAG_UPDATE_SOURCE, 0, peer_change_none},
 	{PEER_FLAG_DISABLE_LINK_BW_ENCODING_IEEE, 0, peer_change_none},
 	{PEER_FLAG_EXTENDED_OPT_PARAMS, 0, peer_change_reset},
@@ -6638,9 +6639,9 @@ int peer_allowas_in_unset(struct peer *peer, afi_t afi, safi_t safi)
 }
 
 int peer_local_as_set(struct peer *peer, as_t as, bool no_prepend,
-		      bool replace_as, const char *as_str)
+		      bool replace_as, bool dual_as, const char *as_str)
 {
-	bool old_no_prepend, old_replace_as;
+	bool old_no_prepend, old_replace_as, old_dual_as;
 	struct bgp *bgp = peer->bgp;
 	struct peer *member;
 	struct listnode *node, *nnode;
@@ -6653,14 +6654,16 @@ int peer_local_as_set(struct peer *peer, as_t as, bool no_prepend,
 		!!CHECK_FLAG(peer->flags, PEER_FLAG_LOCAL_AS_NO_PREPEND);
 	old_replace_as =
 		!!CHECK_FLAG(peer->flags, PEER_FLAG_LOCAL_AS_REPLACE_AS);
+	old_dual_as = !!CHECK_FLAG(peer->flags, PEER_FLAG_DUAL_AS);
 
 	/* Set flag and configuration on peer. */
 	peer_flag_set(peer, PEER_FLAG_LOCAL_AS);
 	peer_flag_modify(peer, PEER_FLAG_LOCAL_AS_NO_PREPEND, no_prepend);
 	peer_flag_modify(peer, PEER_FLAG_LOCAL_AS_REPLACE_AS, replace_as);
+	peer_flag_modify(peer, PEER_FLAG_DUAL_AS, dual_as);
 
-	if (peer->change_local_as == as && old_no_prepend == no_prepend
-	    && old_replace_as == replace_as)
+	if (peer->change_local_as == as && old_no_prepend == no_prepend &&
+	    old_replace_as == replace_as && old_dual_as == dual_as)
 		return 0;
 	peer->change_local_as = as;
 	if (as_str) {
@@ -6689,10 +6692,11 @@ int peer_local_as_set(struct peer *peer, as_t as, bool no_prepend,
 					    PEER_FLAG_LOCAL_AS_NO_PREPEND);
 		old_replace_as = CHECK_FLAG(member->flags,
 					    PEER_FLAG_LOCAL_AS_REPLACE_AS);
-		if (member->change_local_as == as
-		    && CHECK_FLAG(member->flags, PEER_FLAG_LOCAL_AS)
-		    && old_no_prepend == no_prepend
-		    && old_replace_as == replace_as)
+		old_dual_as = !!CHECK_FLAG(member->flags, PEER_FLAG_DUAL_AS);
+		if (member->change_local_as == as &&
+		    CHECK_FLAG(member->flags, PEER_FLAG_LOCAL_AS) &&
+		    old_no_prepend == no_prepend &&
+		    old_replace_as == replace_as && old_dual_as == dual_as)
 			continue;
 
 		/* Set flag and configuration on peer-group member. */
@@ -6701,6 +6705,7 @@ int peer_local_as_set(struct peer *peer, as_t as, bool no_prepend,
 			  no_prepend);
 		COND_FLAG(member->flags, PEER_FLAG_LOCAL_AS_REPLACE_AS,
 			  replace_as);
+		COND_FLAG(member->flags, PEER_FLAG_DUAL_AS, dual_as);
 		member->change_local_as = as;
 		if (as_str)
 			member->change_local_as_pretty = XSTRDUP(MTYPE_BGP_NAME,
@@ -6723,12 +6728,14 @@ int peer_local_as_unset(struct peer *peer)
 		peer_flag_inherit(peer, PEER_FLAG_LOCAL_AS);
 		peer_flag_inherit(peer, PEER_FLAG_LOCAL_AS_NO_PREPEND);
 		peer_flag_inherit(peer, PEER_FLAG_LOCAL_AS_REPLACE_AS);
+		peer_flag_inherit(peer, PEER_FLAG_DUAL_AS);
 		PEER_ATTR_INHERIT(peer, peer->group, change_local_as);
 	} else {
 		/* Otherwise remove flag and configuration from peer. */
 		peer_flag_unset(peer, PEER_FLAG_LOCAL_AS);
 		peer_flag_unset(peer, PEER_FLAG_LOCAL_AS_NO_PREPEND);
 		peer_flag_unset(peer, PEER_FLAG_LOCAL_AS_REPLACE_AS);
+		peer_flag_unset(peer, PEER_FLAG_DUAL_AS);
 		peer->change_local_as = 0;
 		XFREE(MTYPE_BGP_NAME, peer->change_local_as_pretty);
 	}
@@ -6760,6 +6767,7 @@ int peer_local_as_unset(struct peer *peer)
 		UNSET_FLAG(member->flags, PEER_FLAG_LOCAL_AS);
 		UNSET_FLAG(member->flags, PEER_FLAG_LOCAL_AS_NO_PREPEND);
 		UNSET_FLAG(member->flags, PEER_FLAG_LOCAL_AS_REPLACE_AS);
+		UNSET_FLAG(member->flags, PEER_FLAG_DUAL_AS);
 		member->change_local_as = 0;
 		XFREE(MTYPE_BGP_NAME, member->change_local_as_pretty);
 		member->last_reset = PEER_DOWN_LOCAL_AS_CHANGE;
diff --git a/bgpd/bgpd.h b/bgpd/bgpd.h
index def12ee642..795e4fbc58 100644
--- a/bgpd/bgpd.h
+++ b/bgpd/bgpd.h
@@ -1507,6 +1507,7 @@ struct peer {
 #define PEER_FLAG_CAPABILITY_FQDN (1ULL << 37)  /* fqdn capability */
 #define PEER_FLAG_AS_LOOP_DETECTION (1ULL << 38) /* as path loop detection */
 #define PEER_FLAG_EXTENDED_LINK_BANDWIDTH (1ULL << 39)
+#define PEER_FLAG_DUAL_AS		  (1ULL << 40)
 
 	/*
 	 *GR-Disabled mode means unset PEER_FLAG_GRACEFUL_RESTART
@@ -2443,7 +2444,7 @@ extern int peer_allowas_in_set(struct peer *, afi_t, safi_t, int, int);
 extern int peer_allowas_in_unset(struct peer *, afi_t, safi_t);
 
 extern int peer_local_as_set(struct peer *peer, as_t as, bool no_prepend,
-			     bool replace_as, const char *as_str);
+			     bool replace_as, bool dual_as, const char *as_str);
 extern int peer_local_as_unset(struct peer *);
 
 extern int peer_prefix_list_set(struct peer *, afi_t, safi_t, int,