Upon adding a host to an existing Pool configured with NIC Bonding of type LACP, the newly added host's NIC bond entry never finishes updating, as seen on the image below, where the Management interface never merges with the bond. The previous NIC remains as IP Setup None and the bond shows as Unknown interface instead of Management as expected.



Other symptoms include: 

• Connection to host is interrupted and lost.
• xapi commands become unresponsive on newly added host
• An emergency network reset is required to talk to the host again but the connection is lost again briefly after the host comes back up.
• ovs-appctl bond/show bond0 will show 

bond_mode: balance-slb
lacp_status: configured

• But it doesn't show this line:

lacp_negotiated: true

This can happen because the switch configuration for LACP is missing on the switch end. Both types of LACP require configuration on the switch side unlike other types of Bonds like the Active/Active one for instance.

For details on how to configure this on the switch side, refer to the article for LACP Bonding in XenServer - Configuration and Troubleshooting: https://support.citrix.com/article/CTX135690

Note: XenServer gets the LACP setup based on a negotiation with the switch where priority values are passed to XenServer, which is why this might not work when being dynamically configured on the switch side. Using VLT on the switch side is not supported for XenServer LACP bonds for this reason.

LACP Bonding in XenServer - Configuration and Troubleshooting: https://support.citrix.com/article/CTX135690