Centralized DHCP with similar VLANs in multiple offices.

I am doing some work at a client that has 25 offices.  They have a central DHCP server at the main office.  (We won't go into how that is a bad idea right now.)

Each of the 25 offices have the same VLANS, vlan 110 for data, vlan 120 for voice.  Each office has their own 10.x.x.x/16 IP space.

On each of the VLANs there is a helper IP address for the DHCP server pointing back to the central DHCP server.

My question is this --- how does the DHCP server know when it hears a DHCP for VLAN110 in office A to give an IP from the proper scope and not from the scope VLAN 101 of office B?

I suspect the DHCP server somehow determines this from the source IP of the router/switch that has the help IP configured on it local VLAN.  


Thank you,

Scott
BladesAwayAsked:
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Joshua GrantomSenior Systems AdministratorCommented:
Its probably from the router IP at that location and how the DHCP Scopes are setup.
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Ken BooneNetwork ConsultantCommented:
So if each office has a 10.x.x.x/16 I am assuming that the 2nd octet is unique at each location.  Basically the local router that is performing the DHCP relay to the centralized DHCP server has his address on the local segment in the forwarded DHCP packet.  When the DHCP sees this forwarded packet, he knows what network it was forwarded from based on this and provides the appropriate address assignment.  The DHCP has no idea nor does he care what vlan the remote location is on.

Hope that helps.
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BladesAwayAuthor Commented:
That' kind of what I was figuring Ken.  Thank you for confirming.

Scott
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