Summary

mpls inter-as option B require the ability to switching label in the ASBR, it can be done using the following command :mpls bgp l3vpn-multi-domain-switching

Redistribute labeled L3VPN routes from AS to neighboring AS (RFC-4364 option B, or within the same AS when the iBGP peer uses next-hop-self to rewrite the next-hop attribute). The labeled L3VPN routes received on this interface are re-advertised with local labels and an MPLS table swap entry is set to bind the local label to the received label.

[Describe the feature briefly.
Example: "Add support for 802.1q VLAN interfaces."]

Redistribute labeled L3VPN routes from AS to neighboring AS (RFC-4364 option B, or within the same AS when the iBGP peer uses next-hop-self to rewrite the next-hop attribute). The labeled L3VPN routes received on this interface are re-advertised with local labels and an MPLS table swap entry is set to bind the local label to the received label.

Additional information
https://docs.frrouting.org/en/stable-10.6/bgp.html#clicmd-mpls-bgp-l3vpn-multi-domain-switching

under interface option, frr mpls bgp l3vpn-multi-domain-switching

vyos-cli option :

set protocols bgp interface eth1 mpls bgp l3vpn-multi-domain-switching