Man in middle attack in networking

[prince.p]

Hi Friends

i need to clarify some doubt in networking. 

we have Cisco 2900 series router and sonicwall firewall and Cisco switches.

our service provider router is connected with cisco router and cisco router is connected with Sonicwall firewall and in sonicwall firewall we configure 3 vlans interfaces and it connected with cisco switches.

our default gateway to the network is assigned from sonicwall interfaces.

we connected one dlink router in one of the switch vlan1 interfaces. after we connect the dlink, some of the laptop default gateway changed to dlink ip address. so we disable dhcp in dlink router.

i need to know is connected dlink router to cisco switch will cause any network loop or it will create and problem in network.

can anyone help

[Jaderson Pessoa]

Dear, prince.p
If you connect your dlink router, you need disable some feature on it, like dhcp and others. Because if you network doenst working with dhcp snooping, por security or other thing like it, you can have a problem in your network.

You'ill have a problem because dlink has a dhcp service active and you has an other dhcp server active and your ports are the same vlan 1, só all ports will receive dhcp sometime from dlink, sometimes from sonicwall.

[balaji.bandi]

For you Security point of view, Why not create another VLAN and allow only that VLAN for that Dlink users.

Make sure you keep the edge access port in to Acceee mode or L3 intercface, so you can minimise the risk from Dlink. (until you have any reason for this to be L2 link only)