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Configure Server Identifier option Option 54 on a 2012r2 DHCP Server

    Question

  • My network works with Relay Agents on switches with L3 routing. DHCP server in vlan 1, clients in another vlans, routing works fine, but dhcp service only for broadcast requests from clients (request new ip address). Unicast requests from clients (ip address renew operation, command ipconfig /renew) dont work, DHCP server log NACK message, because scopes configured using option 82, but unicast request reseived without option 82 information.

    https://blogs.technet.microsoft.com/teamdhcp/2012/10/01/dhcp-policies-based-on-relay-agent-information-option-option-82-dhcp-snooping-and-ip-source-guard/

    If you hadn’t already noticed, the server identifier override sub-option is not used in policy condition value. Relay agents use this sub-option to provide IP address which should be included by the DHCP server in Server Identifier option [DHCP Option 54] instead of DHCP Server’s IP address in DHCP replies. By default, only the DHCP messages which are broadcast by the client pass via the relay agent. The DHCP renew messages which are unicast by the DHCP client to the IP address of the DHCP server do not pass via the relay agent.

    By inserting the IP address of the relay agent in the server identifier field, the DHCP server ensures that all DHCP requests pass through the DHCP relay agent including DHCP renew messages which will now be unicast to the IP address of the DHCP relay agent. The relay agent can now add option 82 and its sub-options to all DHCP client messages before they are seen by the DHCP server. This ensures the policy is applied for both unicast and broadcast DHCP requests.

    How to configure Option 54 DHCP server?



    • Edited by Tilerans Friday, September 23, 2016 1:06 PM
    Friday, September 23, 2016 11:40 AM

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