Why does a BGP peer send an open message twice on S series switches


The two peers that set up a BGP session are not of the master and slave relationship. Both the two peers attempt to set up a connection. A peer initiatively sends an Open message to set up a BGP connection, and replies with an Open message after receiving the Open message from its peer. During the negotiation, a connection is closed. As a result, a BGP peer may send an Open message twice.

Other related questions:
Why does one BGP end send the open packet twice sometimes
The BGP session relationship is not the master-slave relationship. Both BGP ends may actively establish connections. The open packet is sent regardless of the connection established actively or passively. In addition, one connection is terminated during the negotiation. Therefore, the open packet may be sent twice.

Problem and solution when BGP peer cannot be established
The BGP peer establishment on the firewall needs to use port 179 to establish TCP sessions and requires that OPEN messages be properly exchanged. Perform as follows to rectify the issue: 1. Check whether the AS number and IP address among peers are correct by using the display bgp peer command. 2. Check whether the router IDs configured on both BGP peers are conflicting by using the display bgp peer command. 3. If the loopback interface is used, check whether the peer connect-interface command is configured to specify the loopback interface as the source interface for sending BGP packets. 4. If EBGP neighbors are not directly connected to the physical layer, check whether the peer ebgp-max-hop command is configured. 5. Check whether there are available routes to the peer in the routing table. 6. Check whether there are reachable routes to the specified connect-interface by using the ping -a source-ip-address host-address command. 7. Check whether the ACL that is used to disable TCP port 179 is configured.

Why does an S series switch try to establish a BGP connection over 30s after peer configuration
For S series switches that support BGP, it is more complex to configure BGP than IGP. Besides peers and ASs, outbound interfaces, multiple hops, timers, and various capabilities need to be configured. At present, BGP does not support dynamic capability negotiation. Therefore, the changes of these parameters need to be negotiated again. To prevent the BGP peer relationship from being interrupted frequently, a proper timer is required. Ensure that related configurations are complete before the attempt of setting up a BGP connection. In RFC 4271, you are advised to set the value of the timer to 120s. On Huawei devices, the value of the timer is set to 32s.

In which situation can an S series switch send the option field in Request messages to a client
For S series switches excluding the S1700, the DHCP server returns the corresponding option field to a client only when the corresponding option field is encapsulated in the Request message.

Does an S series switch sends a Report message after receiving a Query message
If only IGMP snooping is enabled, an S series switch does not send Report messages after receiving Query messages. If IGMP snooping proxy or IGMP snooping Report suppression is also configured, the switch can construct and send Report messages according to existing entries after receiving Query messages.

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