I have a host machine with 2 ethernet cards on two different subnets. I've set up bridges for both of them:
In my virtual machines I've connected one virtual ethernet card to br0 and the other to br1. The problem is that only eth1 (in the virtual machine) that is connected to br0 on the host works fully. eth0 that is connected to br1 does not work. I can't connect to machine on the same subnet. I can ping them, but I can't open a tcp connection:

I can tell from tcpdump that the target machine does not see anything from the vm besides an arp request.

The problem is the same in the opposite direction. I can ping the vm from an entirely different network, but if I try to telnet to it tcpdump shows me that the vm never sees anything.

So the question is why does one virtual ethernet card work and not the other? On the host I compared the files in /proc/sys/net/ipv4/conf and they are exactly the same for eth0/eth1 br0/br1 tap0/tap1

The command line for my vm (started by libvirtd) is :

/usr/bin/qemu-kvm -S -M pc -m 2048 -smp 3 -name dmail -uuid <snip> -monitor pty -pidfile /var/run/libvirt/qemu//dmail.pid -boot c -drive file=/var/lib/libvirt/images/dmail.img,if=ide,index=0,boot=on -drive file=/var/lib/libvirt/images/dmail-b.img,if=ide,index=1 -drive file=/local/download/CentOS-5.3-x86_64-bin-DVD/CentOS-5.3-x86_64-bin-DVD.iso,if=ide,media=cdrom,index=2 -net nic,macaddr=54:52:00:11:cb:ae,vlan=0 -net tap,fd=22,script=,vlan=0,ifname=vnet4 -net nic,macaddr=54:52:00:11:cb:af,vlan=1 -net tap,fd=23,script=,vlan=1,ifname=vnet5 -serial pty -parallel none -usb -vnc 127.0.0.1:2 -k en-us