If we have a v6 neighbor that should only exchange v6 routes, it seems that
set protocols bgp parameters default no-ipv4-unicast
should do it. But,
vtysh show ip bgp ipv4 sum Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd PfxSnt 205.147.6.21 4 7397 134808 26 0 0 0 00:22:07 856222 1 2001:470:1f00:1a::1 4 6939 35758 26 0 0 0 00:22:07 NoNeg 2604:0:b:4::1 4 7397 33958 26 0 0 0 00:22:07 NoNeg show running-config address-family ipv4 unicast network 205.147.40.32/27 neighbor 205.147.6.21 activate neighbor 205.147.6.21 soft-reconfiguration inbound neighbor 205.147.6.21 route-map local-nets-filter out neighbor 2001:470:1f00:1a::1 activate neighbor 2604:0:b:4::1 activate exit-address-family
We should have no neighbor ... activate for ipv6 neighbors in the address-family ipv4 unicast block. Unless I am misunterstanding the meaning of default no-ipv4-unicast.