So, the problem is: I was trying to set a Speed Touch on corporate network, not for internet access but just for tech support to end users. Configuration was in this shape: Bridge mode + DHCP off + IP/subnet/default gateway of our corporate network(10.2.45.192/255.255.255.0/10.2.45.1). All computers of tech support are on different VLAN 10.2.43.~ Static routes were also defined OK. So anytime i connected a router on LAN, he started a DHCP and tryed to assign IP adresses to all devices on the 10.2.45.~ LAN ( computers, Avaya IP phones...), and whole LAN falt down. THis happened 3 times. What's wrong with this router? I connected a 3 other routers ( Huawei MT882, Huawei HG 510 and Sagem 1241) and thay are working good.