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    RobD's Avatar
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    Lightbulb DHCP, default Gateways and vlan fun

    Hi All,

    I have a few questions on VLANs, default gateways and DHCP:

    We have three switches at our primary school, traditionally all three switches had a default gateway of the core switch which is on a different site and is connected via a leased line.

    Each vlan had a IP of its own which was located on the core switch on the main school site. The reason being so we could have a DHCP scope for each vlan, so then on the core switch we would have a ip helper which would point to our DHCP server.

    This all worked and was fine except if we had an issue at the main site the primary school would also suffer.

    So to combat this I added DHCP to the local DC and configured the scopes, changed the default gateway on the primary schools switches to the switch the DC was plugged into (call it 84) and added a IP helper to point at the local DC and added the IP address to the vlan. Then deleted the old scopes and vlans at the main site.

    So in theory the PC would look at its local switch 110, the switch would then push all traffic to the switch 84 (default route via 85), switch 84 would use the IP helper to direct the PC to the DC and get a IP (see diagram). DHCP.jpg

    Unfortunately this isnít working, the PCís arenít getting IP addresses as they seem to be trying to look at the core switch located on the main site (as they used to) instead of locally. Iím not sure where Iím going wrong here or what to check. The vlan tagging seems ok, the PC's are untagged and the route through to the servers is taggged. Any ideas what I should look at?

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    robk's Avatar
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    Have you enabled ip routing on one of the local switches?

    My guess is that the core switch had routing info for all the vlans, which you need to add to one of the local switches (maybe the one with the dc's that you have set as default route) to get things going.

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    RobD's Avatar
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    Hi, thanks for the reply.

    IP Routing is enabled on all the switches (is that correct?), looking "show IP Route" the vlan only appears where the IP is configured i.e. on the switch that has my DC on.

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    Could this be an ACL some where? by default VLAN do you mean that you have got your servers on the management VLAN of your switches? or do you have a default server VLAN setup? anyhow that aside packet trace to see what's going on and where. Another solution is to post the config of your switches (Core as well as edge) so that one of us can have a quick look.

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    RobD's Avatar
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    Hi All,

    Thanks for the replies, Really helpful.
    Unfortunately for me the problem ended up being really quite straight forward. The default gateway on the DC on site2 was the core switch on site1 which meant I couldn't route to the vlan. I changed the default gateway on the DC to the local core switch and bobs your uncle it all worked. Thanks again.



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