How to make two vlans on one interface working with dhclient

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Hi,
I have a FreeBSD 6.0 box andi need to configure it in a way that I must use
2 vlans under one interface and both of them will get an address thanks to
dhclient.

So in order to make this job I created vlans:

 ifconfig rl0.100 create
ifconfig rl0.2101 create
The vlans are created without any problem.
But I also want to retrieve an diffrent ip address from a dhcpd server on
the two of them.
I've made a specific dhclient.conf for each one:

interface "rl0.100" {
#My options
}
interface "rl0.2101" {
#My options
}

When I launch dhclient on only one interface (100 or 2101) it's works fine
and I get an ip address (class B on vlan 100 and class A on vlan 2101).
But if I launch dhclient on the second interface it's make an error:

dhclient: Can't bind to dhcp address: Address already in use. Please make
sure there is no other dhcp server runing and that there's no entry for dhcp
or bootp in /etc/inetd.conf

Does someone had an idea about how to fix this problem and having my two
vlans working with dhclient?