--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -1896,7 +1896,12 @@ the BFD session went down).
 advanced features like the echo mode or authentication are not implemented), IP
 transport for BFD as defined in <rfc id="5881"> and <rfc id="5883"> and
 interaction with client protocols as defined in <rfc id="5882">.
-We currently support at most one protocol instance.
+
+<p>As BFD protocol implementation binds a wildcard socket, at most one protocol
+instance per VRF is supported. Note, that Linux kernel versions below 5.0 do
+not allow simultaneous wildcard sockets in several VRFs. In that case you can
+use single BFD instance without VRF with sysctl option set:
+<cf>net.ipv4.udp_l3mdev_accept=1</cf>
 
 <p>BFD packets are sent with a dynamic source port number. Linux systems use by
 default a bit different dynamic port range than the IANA approved one