Why is the BGP connection not interrupted immediately after the interfaces connecting two peers are shut down

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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.

Other related questions:
Why can BGP not immediately attempt to establish connections after the peer is configured
To prevent the Border Gateway Protocol (BGP) from frequently tearing down the neighbor relationship for renegotiation, the device waits for a proper period before establishing connections to ensure that related configurations are complete. The recommended period in RFC4271 is 120 seconds. The implementation in AR series routers is 32 seconds.

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.

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

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