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If a device has a dhcp pool defined:

ip dhcp pool DATA
  network 192.0.2.0 255.255.255.0
  default-router 192.0.2.1
  domain-name test.net

But it also has a DHCP helper setup on the interface:

interface GigabitEthernet0/1
  ip address 192.0.2.1 255.255.255.0
  ip helper-address 198.51.100.10

Then what is the result? Is the DHCP addresses given from the helper? or from the local pool?

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  • I would have to lab this out to be sure, but I believe you will get an offer from both the local pool and from the DHCP server 198.51.100.10 (if it exists). If I get a chance later, I will try to run this in my lab to confirm.
    – YLearn
    Jun 13, 2014 at 14:52

1 Answer 1

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4

Is the DHCP addresses given from the helper? or from the local pool?

You would get two possible DHCPOffer's from the device's internal DHCP pool and the DHCP server defined by the helper-address, but it's up to the client to decide which DHCPOffer to accept and to then unicast a DHCPRequest to the DHCP server of choice.

Often clients choose the DHCP server with the fastest DHCPOffer. Unless your device's CPU is bogged down, I would expect typical clients to select the DHCP pool more often than not. You'd expect a little less latency in a L2 operation vs L3 which is required to get to the DHCP server via the helper-address.

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  • 1
    Given there isn't anything wrong with the DHCP packet, it always comes down to the fastest response.
    – Ryan Foley
    Jun 15, 2014 at 14:36

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