Mutiple Cisco Switches doing DHCP with different default gateways - does this work?

Posted on 2009-04-29
Last Modified: 2012-05-06
Presently we have two Layer 3 Cisco switches doing DHCP for our network, with a shared HSRP address.

We are migrating to a Gigabit infrastructure - I have installed 2 new Layer 3 Cisco switches & migrated half our infrsatructure to these new switches.  The old switches are connectesd to the new switches (presently a single Etherchannel but will soon be full mesh)

I want devices that are connected to these new switches to be given the HSRP address of these switches as their default gateway, & devices that are connected to the old switches to still get the HSRP address of these switches as their default gateway.

At the moment, traffic between subnets has to go via the old switches, even if both devices are connected to the new switches (as all devices have a default gateway of the old switches - I believe so anyway; when I do a trace I see the IP of the old switches)

It is easy enough to configure, but with the way DHCP works, I don't want devices on one side being given the default gateway for the other side.  I would hope that when the new switches receive a DHCP request, they will simply reply to it & not send it on to the old switches, & vice versa.  So the switches will only receive DHCP requests for their side, & will not even receive requests for the wrong side.

Does anyone have this setup & can they advise if it works no problem?  

Please see diagram: example topology - devices connected to the switches on the left should get a default gateway of .1, & devices on the right should get a gateway of .100.  It makes sense that it would work this way - if a switch is a DHCP server it will not propagate the requests.?)

Any questions please ask.

Many thanks!

Question by:Sir_Lambalot
    LVL 3

    Expert Comment

    What is the HSRP address? Is this the address you want to be the Def Gtwy for all PCs, correct?
    Which switches are handling your inter-VLAN routing? Are there VTP domains anywhere in your diagram?
    I'm going to have to process this a little more to try and better understand what you're trying to accomplish.

    Author Comment

    Hi Ken - thaks for replying.  Sorry - the diagram is a little confusing - Layer3 A is supposed to represent 2 switches & Layer3 B represents 2 switches as well.  The IPs that I've shown for them represent the HSRP addresses for the pairs.

    On our LAN we have a number of VLANs, so both pairs of switches do inter-VLAN routing.  Layer3 B points to Layer3 A as it's default route, if traffic needs to go out onto the internet for example.  Switches are in the same VTP domain.

    At the moment, 2 devices connected to Layer3 B that are in different VLANs have the Layer3 A HSRP address for the relevant VLAN as their default gateway.  If I do a trace between the devices it seems that traffic goes via Layer3A when it doesn't need to.  So I want to set up the Layer3 B pair of switches to do DHCP for that side of the network, hand out the Layer3 B HSRP address for the relevant VLAN as the default gateway & traffic no longer needs to go to the Layer3 A pair of switches.

    I just want to confirm that devices connected to the Layer3 B pair of switches get the Layer3 B HSRP address, & those connected to Layer3 A still get the Layer3 A HSRP address.  I would have thought so - any DHCP request from the right hand side has to go via the Layer3B pair of switches, and as the Layer3 B pair are doing DHCP they should reply in all cases with an IP, default gateway, etc, rather than ever forwarding the DHCP request to the Layer3 A pair of switches.

    Do I need to allocate different pools of addresses to the different switches?  I think that would make sense (at the moment the Layer3 A pair distribure IPs from the same pool, but will I ever get duplicate IPs handed out if I'm using 4 switches for DHCP they're handing out different default gateways?)

    Many thanks for your help!


    Accepted Solution

    I'm pretty sure it will work OK
    LVL 3

    Expert Comment

    I'm sorry I couldn't add anything more. Feel free to update your question and let us know how it goes.


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