DHCP Issues With Catalyst 3560G

Answered Question
Oct 15th, 2008

I have my local network hooked to a vlan with a primary and secondary ip address assigned to the vlan. I have manual DHCP bindings setup in the switch using the MAC address of my laptops for the client identifier. Some laptops aren't pulling addresses and I am not seeing any message sent from the DHCP server in the switch stating NAKs, Offers, or Requests. Should DHCP work correctly even if there is a secondary IP address on the VLAN?

I have this problem too.
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Correct Answer by Edison Ortiz about 8 years 1 month ago

So would I be correct in assuming the secondary IP address would be causing the problem?

Yes, it's not a supported configuration for translating DHCP broadcast traffic into unicast towards the DHCP server located in a different Vlan.

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Edison.

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Edison Ortiz Wed, 10/15/2008 - 08:33

I suggest re-designing your network. How would the Vlan know the source of the broadcast needs to use the secondary or primary IP on the Vlan to unicast this information to the DHCP server?

The initial DHCP query is sent as broadcast from the client and the Vlan transform this query to unicast with the ip helper-address to the DHCP server. The source of the unicast will always be the primary IP address so the DHCP server will respond from the scope with the primary IP address associated as the router from that Vlan. It will never use the secondary IP address as it will never receive an unicast packet with that sort of information.

HTH,

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Edison.

tom.gabriel Wed, 10/15/2008 - 11:32

The secondary IP on the VLAN is temporary until we can change all devices on the network. This is not the normal way we have set the VLAN up. So would I be correct in assuming the secondary IP address would be causing the problem?

Correct Answer
Edison Ortiz Wed, 10/15/2008 - 11:56

So would I be correct in assuming the secondary IP address would be causing the problem?

Yes, it's not a supported configuration for translating DHCP broadcast traffic into unicast towards the DHCP server located in a different Vlan.

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Edison.

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