why ng_bridge does not like DHCP?

[noname]

 freebsd 5.4 stable. xl0 is connected via ng_bridge to ngeth0 in this way:

[root at asd:~]# ifconfig xl0 up
[root at asd:~]# ngctl mkpeer . eiface hook ether
[root at asd:~]# ifconfig ngeth0 up
[root at asd:~]# ngctl mkpeer xl0: bridge lower link0
[root at asd:~]# ngctl name xl0:lower mybridge
[root at asd:~]# ngctl connect ngeth0: mybridge: lower link1
[root at asd:~]# ngctl connect ngeth0: mybridge: upper link2
[root at asd:~]# ngctl msg xl0: setautosrc 0
[root at asd:~]# ngctl msg xl0: setpromisc 1
[root at asd:~]# ifconfig ngeth0 ether 00:12:12:12:12:12
[root at asd:~]# dhclient ngeth0

using tcpdump I can see dhcp request leaving from ngeth0, passing
through xl0, dhcp reply comes back through xl0 but it doesn't reach
ngeth0. Why?

If I give ip-address with ifconfig, everything works fine. Also
getting ip with dhcp to xl0, without touching netgraph works fine. Any
clues?