Why are BGP connections reestablished after the peer connect-interface command is executed on S series switches

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Currently, S series switches do not support BGP dynamic capability negotiation. Therefore, when capabilities of the BGP neighbor are changed, the BGP connection is automatically cut off and then the capabilities of the neighbor are renegotiated. If the peer connect-interface command is configured, the BGP session needs to be set up by designating the egress. Therefore, the source address of the TCP connection may be changed and the TCP connection needs to be re-established by using the new source address.

Other related questions:
For S series switches, why is the BGP connection not immediately closed after the shutdown command is executed on the interfaces connecting two peers
Q: Why is the BGP connection not cut off immediately after the shutdown command is executed on interfaces connecting two devices? A: When the EBGP peers are directly connected and the ebgp-interface-sensitive command is executed in the BGP view, the BGP peer connection is interrupted immediately after the interfaces connecting the two peers are shut down. By default, the ebgp-interface-sensitive command is executed in the BGP view. In other cases, the BGP peer relationship is not interrupted until the Hold timer times out.

Why are BGP connections re-established after the peer connect-interface command is configured
Currently, the switch does not support BGP dynamic capability negotiation. Therefore, if certain capabilities of the BGP peer are changed, the BGP connection is automatically disconnected and then the capabilities of the neighbor are renegotiated. If the peer connect-interface command is configured, the BGP session needs to be set up by designating the egress. Therefore, the source address of the TCP connection may be changed and the TCP connection needs to be re-established by using the new source address.

On an S series switch, after the reset bgp ipv4-address command is executed to reset BGP connections using the IPv4 unicast address family, why is the connection of BGP peers using the VPNv4 address family terminated
Q: After the reset bgp ipv4-address command is executed to reset the BGP connection of the unicast address family, why is the BGP connection of the same peers in the VPNv4 address family cut off? A: If the peers of two address families have the same addresses, the two address families share the same BGP session, that is, the same TCP connection. If one of the BGP connections is reset, the other is disconnected.

Why is the BGP connection not interrupted immediately after the interfaces connecting two peers are shut down
When External Border Gateway Protocol (EBGP) peers are directly connected and the ebgp-interface-sensitive command is run in the Border Gateway Protocol (BGP) view, the BGP peer relationship is interrupted immediately after the interfaces connecting the two peers are shut down. By default, the ebgp-interface-sensitive command is run in the BGP view. In other cases, the BGP peer relationship is not interrupted until the Hold timer times out.

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.

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