Merge pull request FRRouting#19298 from donaldsharp/bgp_dual_as_fixes

Bgp dual as fixes

Author: Jafaral

bgpd/bgp_packet.c

@@ -1654,6 +1654,9 @@ static int bgp_collision_detect(struct peer_connection *connection,
 	 */
 	if (peer_established(other) ||
 	    other->status == Clearing) {
+		if (bgp_debug_neighbor_events(peer))
+			zlog_debug("New connection starting from %pI4 but a competing connection is already in established or clearing",
+				   &remote_id.s_addr);
 		bgp_notify_send(connection, BGP_NOTIFY_CEASE,
 				BGP_NOTIFY_CEASE_COLLISION_RESOLUTION);
 		return -1;
@@ -1691,6 +1694,9 @@ static int bgp_collision_detect(struct peer_connection *connection,
 					BGP_NOTIFY_CEASE_COLLISION_RESOLUTION);
 			return 1;
 		} else {
+			if (bgp_debug_neighbor_events(peer))
+				zlog_debug("New Connection received, but existing connection from %pI4 will win collision detection so dropping this new connection",
+					   &remote_id.s_addr);
 			bgp_notify_send(connection, BGP_NOTIFY_CEASE,
 					BGP_NOTIFY_CEASE_COLLISION_RESOLUTION);
 			return -1;

doc/user/bgp.rst

@@ -1877,7 +1877,10 @@ Configuring Peers
 
    The ``dual-as`` keyword is used to configure the neighbor to establish a peering
    session using the real autonomous-system number (``router bgp ASN``) or by using
-   the autonomous system number configured with the ``local-as``.
+   the autonomous system number configured with the ``local-as``.  If ``dual-as`` is
+   used be aware of connection collision ordering and attempt to configure this system
+   with a higher ip address so that this connection is preferred.  As that the other
+   side will reject the incoming connection.
 
    This command is only allowed for eBGP peers.
 

tests/topotests/bgp_dual_as/r1/frr.conf

@@ -1,6 +1,6 @@
 !
 interface r1-eth0
- ip address 10.0.0.1/24
+ ip address 10.0.0.3/24
 !
 router bgp 65000
  no bgp ebgp-requires-policy

tests/topotests/bgp_dual_as/r2/frr.conf

@@ -4,7 +4,7 @@ interface r2-eth0
 !
 router bgp 65002
  no bgp ebgp-requires-policy
- neighbor 10.0.0.1 remote-as 65000
- neighbor 10.0.0.1 timers 3 10
- neighbor 10.0.0.1 timers connect 1
+ neighbor 10.0.0.3 remote-as 65000
+ neighbor 10.0.0.3 timers 3 10
+ neighbor 10.0.0.3 timers connect 1
 !