vyos@vyos# comp
[]
+ protocols {
+     bgp {
+         address-family {
+             ipv4-unicast {
+                 redistribute {
+                     connected { }
+                 }
+             }
+             ipv6-unicast {
+                 redistribute {
+                     connected { }
+                 }
+             }
+         }
+         neighbor eth1 {
+             interface {
+                 v6only {
+                     peer-group "fabric"
+                 }
+             }
+         }
+         neighbor eth2 {
+             interface {
+                 v6only {
+                     peer-group "fabric"
+                 }
+             }
+         }
+         parameters {
+             bestpath {
+                 as-path {
+                     multipath-relax { }
+                 }
+                 compare-routerid { }
+             }
+             router-id "192.168.0.1"
+         }
+         peer-group fabric {
+             address-family {
+                 ipv4-unicast { }
+                 ipv6-unicast { }
+             }
+             capability {
+                 extended-nexthop { }
+             }
+             remote-as "external"
+         }
+         system-as "64496"
+     }
+ }

[edit]
vyos@vyos# commit

Neighbor "eth1" remote-as must be set!