I have a transparent ADSL modem (the modem does not have assigned an IP
address from the provider but forwards all traffic to my FreeBSD box,
which owns the actual IP address). 

For mgmt purposes I access the modem from the FreeBSD box using the
modems 10.0.0.138 IP address. Because the ISP IP address is assigned to
the FreeBSD box, the FreeBSD box does not have an address in 10.0.0.0/24.

To make this work I use the following command to create 

route add 10.0.0.138 -netmask 255.255.255.255 -interface -link xl2:00.0e.50.23.5e.13

This works perfectly, a host entry is created in the routing table and
the link layer address is added as well:

# netstat -nr | grep 10.0.0.138
10.0.0.138/32      00:0e:50:23:5e:13  ULS         5     5370    xl2


The problem is that it does not work if a vlan interface is used instead
of a physical interface:

# ifconfig em0.99 create
# route add 1.2.3.4 -netmask 255.255.255.255 -interface -link em0.99:00.0e.50.23.5e.14
add net 1.2.3.4: gateway em0.99:00.0e.50.23.5e.14
# netstat -nr |grep 1.2.3.4
1.2.3.4/32         em0:99.0.e.50.23.5e.14 US          0        0    em0

As you can see, something seems to go wrong with the parsing. This problem
also exists in FreeBSD 6.x

Fix: 

No fix known.
How-To-Repeat: # ifconfig em0.99 create
# route add 1.2.3.4 -netmask 255.255.255.255 -interface -link em0.99:00.0e.50.23.5e.14
add net 1.2.3.4: gateway em0.99:00.0e.50.23.5e.14
# netstat -nr |grep 1.2.3.4
1.2.3.4/32         em0:99.0.e.50.23.5e.14 US          0        0    em0