All of my neighbors are connected with me via wireguard interfaces (a different interface for every peering). I have no physical direct link with any peer.
All neighbors using IPv4 or ULA IPv6 addresses are working properly.

i was referring to the FRR command as deprecated, not the corresponding VyOS command. The VyOS command is not even available in the last version of VyOS... I was able to try it only via vtysh...

upgraded to 1.4-rolling-202106290839 but still not working for my setup...

looking at your configuration I see you set the neighbor using the interface name.
But in that case how does FFR know which IP address to connect to initiate a BGP session? Works in passive mode only?

In T3657#97243, @c-po wrote:

Also FRR manual states:

When you connect to a BGP peer over an IPv6 link-local address, you have to specify the IFNAME of the interface used for the connection. T

Please try set protocols bgp neighbor <addr> interface eth1

Hi, any updates on this?

In T3117#81576, @c-po wrote:

Unfortunately setting udp4 was never "valid" in the first place, this was only possible by a wrong regex here: https://github.com/vyos/vyos-1x/blob/406083932ae62ccde5ff547ef7d7960efe0269e3/interface-definitions/interfaces-openvpn.xml.in#L345 this has been corrected already which shows the above mentioned result in a not loading config.