problem/misunderstanding concerning a virtual bridge

Hello everyone,

I'm running several UML (user-mode-linux) guests on a colocated
server. I use the kernel virtual bridge support to make each
guest have direct access to the network, via a tuntap device.

I'm having trouble assigning a second IP address to one of the
guests and I'm at a loss to explain what's wrong. The
configuration looks something like this:

Host Guest Instances
eth0 --- [ bridge ]
| tap0 | ---> UML Guest 0
| tap1 | ---> UML Guest 1
| ... |
| tapn | ---> UML Guest N

I'm trying to assign two IP addresses to guest0; each IP is in a
separate subnet. The guest in question has an existing IP
address from subnet X, which is working fine. I tried setting up
the second IP address, from subnet Y, as both an alias and a
separate tuntap device--both failed in the same manner.

Clues:

1) setting up the new IP address on the host works as
expected. (i.e., I can ping the IP externally and from the UML
guests.)
2) when the ip is configured inside the guest, even the host
machine can not ping the newly configured IP.
3) when the IP is configured as its own tuntap interface, the
only traffic seen on the interface is the following:

09:57:14.896486 802.1d config 8129.00:0f:f7:79:bb:c0.8015 root \
8129.00:0f:f7:79:bb:c0 pathcost 0 age 0 max 20 hello 2 fdelay 15


Thank you VERY much in advance for any hints as to what might be
wrong here,

-majere