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Wired Networks Thread, Easy flat network config... in Technical; Or it should be. Pictures are good. Here's one I made earlier. Shouldn't be able to go far wrong. No ...
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    synaesthesia's Avatar
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    Easy flat network config...

    Or it should be.

    Pictures are good. Here's one I made earlier.


    Shouldn't be able to go far wrong. No vlans. 2 DHCP servers dishing out slightly different ranges (got one range f.ex 192.168.5.0/22 split into 2, 192.168.5.1-192.168.7.254 and 192.168.8.1 to 192.168.10.254)
    Need Site A workstations to pick up DHCP from Site A DHCP server and B stations to pick up from B. All stations need to talk to eachother.

    I read somewhere ages ago that 2008 should just handle this anyway based on speed of the relevant DHCP servers. Doesn't work, the two are different buildings but connected via gig fibre. No routing so no extra hops.
    Tried using an ip helper-address on the switches (procurve 5406zls), doesn't make a difference. Enabled routing on them (just ip routing enabled) with the default route on each set to 0.0.0.0 0.0.0.0 192.168.5.1 (gateway router address).
    No difference, machines still pick up IP's from each others servers.
    Do I need to go any more complicated with the routing setup? Even though I cleared DHCP leases from the servers would the clients still "remember" their last IP and connect to that as preferred?

  2. #2


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    The only way you could do it as it stands is to block DHCP traffic at the router.

    Why would you want 2 DHCP servers on one network? Just have 1, the buildings are connected by 1gb fibre after all.

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    synaesthesia's Avatar
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    Mostly so I can keep traffic off that fibre and to keep each site separate with their relevant IP subnets - so when software is deployed it's done so via the right server rather than murdering that link unnecessarily.
    Plus failover, resiliency etc. Annoyingly we're not allowed to touch the router (twitches uneasily until Capita beggar off)

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    Can you take routing or firewalling from the router and onto the L3 switches? (Is there actually 2 links into the router, can you not make it 1 with the L3 switches attached to one another?)
    As above you can then firewall the ports used (doubt you can without routing tbh). Or if that's not possible, do a fully routed solution with 2 different subnets and turn ip-helper off.

    The reason I suspect ip-helper isn't having the desired effect is the switch knows the dhcp server is on the same subnet as the requests coming in and knows not to route anything... Which could possibly be used to your advantage with a bit of bodging.
    Last edited by j17sparky; 13th August 2012 at 11:07 PM.

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    teejay's Avatar
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    Basically you need to put each side in a separate VLAN and set up routing betwenn the 2 VLANS on the L3 switches. Don't set an ip helper address as your DHCP server you want to use will be within the VLAN.

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    Quote Originally Posted by synaesthesia View Post
    Or it should be.

    Pictures are good. Here's one I made earlier.


    Shouldn't be able to go far wrong. No vlans. 2 DHCP servers dishing out slightly different ranges (got one range f.ex 192.168.5.0/22 split into 2, 192.168.5.1-192.168.7.254 and 192.168.8.1 to 192.168.10.254)
    Need Site A workstations to pick up DHCP from Site A DHCP server and B stations to pick up from B. All stations need to talk to eachother.

    I read somewhere ages ago that 2008 should just handle this anyway based on speed of the relevant DHCP servers. Doesn't work, the two are different buildings but connected via gig fibre. No routing so no extra hops.
    Tried using an ip helper-address on the switches (procurve 5406zls), doesn't make a difference. Enabled routing on them (just ip routing enabled) with the default route on each set to 0.0.0.0 0.0.0.0 192.168.5.1 (gateway router address).
    No difference, machines still pick up IP's from each others servers.
    Do I need to go any more complicated with the routing setup? Even though I cleared DHCP leases from the servers would the clients still "remember" their last IP and connect to that as preferred?
    i see the split scope option as a method to load balance and provide a level of fault tolerance, if you want to direct a client to particular dhcp server, i'm not aware of a metric you can inject at either the l2 or l3 that will determine the 'nearest' dhcp for a flat switched network like yours. i don't see ip routing or ip helper being of any use if your clients exist only in the default vans. Best bet is to split the two sites by vlan and then implement ip routing between the vlans. And then see how you fare with ip-helper if you want to persist with it.

  7. #7

    synaesthesia's Avatar
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    Aye, we tried exactly that last year and failed spectacularly. Will probably try that again in october half term.

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    Ric_'s Avatar
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    As others have touched on... you need two completely separate subnets (not one split in half) and then your DHCP will work on each site.

    Now you will need to make sure all the routing is correct on your two switches. The two switches will need to be physically connected together though since you can't use the gateway for routing between the two (since you have no permission to configure it).

    Then you will need to set up your sites and services in AD appropriately so that things work.

  9. #9

    sonofsanta's Avatar
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    Could you not do anything with the Delay Configuration on the Advanced tab of Scope properties? It may not work all that reliably if the link is fast enough, though, VLANning would probably still be the only way to be certain.

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    Duke5A's Avatar
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    You're going to have to split the subnet up; here is a way to do it while still maintaining the resiliency of having two DHCP servers and keeping the sites workstations identifiable by addressing:

    • Break each site into it's own VLAN with a 23 bit subnet
    • Create scopes for both of the site's subnets on each DC
    • Limit the addresses to half of each scope on each respective DC
    • Add IP helper addresses pointing to the other DC on the router


    Workstations will pull site specific addresses from each DC and you'll be limiting the broadcast domain to each site.

    network.jpg
    Last edited by Duke5A; 20th August 2012 at 11:22 PM.

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