bridge name     bridge id               STP enabled     interfaces
br0             8000.00144fa1f17a       no              eth0
                                                        tap0
                                                        tap1
                                                        tap12
                                                        tap14
                                                        tap16
                                                        tap18
br1             8000.deadbeef3200       no              tap11
                                                        tap13
                                                        tap15
                                                        tap17
                                                        tap9

Now I want to bring up a VM with two NICs, one attached to tap12
(bridge 0), the other on tap11 (bridge 1), but I think I've
misunderstood the versious "-net" options as described in the kvm man
page.  It *seems* to say that the way to map a specific tap to a
specific guest NIC is to say, "-net nic" followed by "-net
tap,name=<name of the tap from the host OS's perspective>", and repeat
that sequence for each nic to appear within the VM.

So, here's the full command line I tried, based on that reading:

 # kvm -m 512M -net nic -net tap,name=tap11 -net nic -net
tap,name=tap12 -hda /vmstore/wee -vnc :11 -cdrom
/path/to/my/Windows.iso -boot d

But when I try that, the host OS brings up two new tap interfaces
(tap2 and tap3), the guest OS never boots (at least, nothing pops up
on its console through VNC), and networking on my system completely
freezes up until I kill -9 the kvm process (kill -15 is ignored) and
restart br0.

I tried "fd=" instead of "name=", but that looks for a file descriptor
instead of a network interface name, and I didn't even know that Linux
had file descriptors for network interfaces let alone how to map them
to a tap.  Nothing under /dev looks promising.

You need to add a vlan option to one of them, for example vlan=2
Otherwise kvm will bridge the interfaces together, and it's going to create a packet storm.
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