Finally, I managed to solve the problem. The problem was rather strange -
 after configuring bridges I added new ifaces to the VM. For some 
reason, during boot process of the VM's, wrong interface was started (VM
 tried to bring up eth2 instead eth1?).<br><br>After adding the new 
ifaces through virt-manager, the VM started regulary and created a new 
network script for eth2 (/etc/sysconfig/network-scripts/ifcfg-eth2). I 
think that the problem was this - when the VM is restarted it tried to 
bring up eth2 while vif-script tried to propagate second bridge via eth1
 iface. I resolved this simply by replacing ifcfg-eth2 script with 
ifcfg-eth1. Now, VM starts/restarts regulary ;)<br><br>So, the process for creating and configuring new bridges that I follow is (tried on multiple servers):<br><br>- create new script that sets up multiple bridges in /etc/xen/scripts/<br>
<br>example: network-xen-multi-bridge<br><br><div class="xoopsCode"><code><pre>#!/bin/sh
# network-xen-multi-bridge
# Exit if anything goes wrong.
set -e
# First arg is the operation.
OP=$1
shift
script=/etc/xen/scripts/network-bridge.xen
case ${OP} in
start)
        $script start vifnum=3 bridge=xenbr3 netdev=eth3
        $script start vifnum=0 bridge=xenbr0 netdev=eth0
        ;;
stop)
        $script stop vifnum=3 bridge=xenbr3 netdev=eth3
        $script stop vifnum=0 bridge=xenbr0 netdev=eth0
        ;;
status)
        $script status vifnum=3 bridge=xenbr3 netdev=eth3
        $script status vifnum=0 bridge=xenbr0 netdev=eth0
        ;;
*)
        echo &#39;Unknown command: &#39; ${OP}
        echo &#39;Valid commands are: start, stop, status&#39;
        exit 1
esac
</pre></code></div><br><br>- make it executable (chmod 755 network-xen-multi-bridge)<br><br>- configure /etc/xen/xend-config.sxp (comment out the current script and add the new one);<br><br><div class="xoopsCode"><code><pre>
# It is possible to use the network-bridge script in more complicated
# scenarios, such as having two outgoing interfaces, with two bridges, and
# two fake interfaces per guest domain.  To do things like this, write
# yourself a wrapper script, and call network-bridge from it, as appropriate.
#
#(network-script network-bridge)
(network-script  network-xen-multi-bridge)
</pre></code></div><br><br>- restart physical server so the bridges are properly created<br><br>- add virt. ifaces to the VM&#39;s (via virt-manager or via config. files in /etc/xen/ ..)<br><br>- boot VM<br><br>- configure newly added iface in /etc/sysconfig/network-scripts/<br>
<br>- restart VM<br><br>Additionaly I check everything by restarting phys. server to make sure that everything is ok ;)