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Can you please explain what needs to be done in order for a computer in one VLAN to get an IP from a DHCP server in another/different VLAN?

(Yes if something is done and no if that something is not done is all I know)

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  • why you add vlan when you want to be in lan to use dhcp ?
    – YuKYuK
    Apr 21, 2016 at 8:44

1 Answer 1

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when a client requests an IP address it does not know anything about the DHCP server so it broadcasts a DHCP DISCOVER packet to all hosts on its segment, if it does not get a response - and it won't in your case - DHCP won't work, in order to avoid setting up an DHCP server in every VLAN we use 'IP HELPER' address, this is a switch configuration that detects DHCP DISCOVER and answers with the IP of the DHCP server.

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_dhcp/configuration/12-4t/dhcp-12-4t-book/config-dhcp-relay-agent.html

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  • we use 'IP HELPER' address, this is a switch configuration that detects DHCP DISCOVER and answers with the IP of the DHCP server - That's not correct. A DHCP Relay Agent forwards the DHCP broadcast traffic as unicast traffic to the DHCP server on behalf of the DHCP client. The DHCP Relay Agent does not answer the client with the ip address of the DHCP server.
    – joeqwerty
    Apr 21, 2016 at 15:36
  • correct, - it doesn't actually change the DHCP protocol, the answer comes from the DHCP server via the switches 'pass it on' config.
    – Sum1sAdmin
    Apr 21, 2016 at 15:49
  • ok so a DHCP Relay Agent forwards the DHCP broadcast traffic as unicast traffic to the DHCP server on behalf of the DHCP client, and how does the DHCP client gets an answer (or the IP address) then? Does the Relay Agent forward the answer to the client from the server?
    – A Fahad
    Apr 24, 2016 at 4:32

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