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Wireless AP's not getting DHCP addresses, but other devices get DHCP with no problem with DHCP Server Cluster

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TitleWireless AP's not getting DHCP addresses, but other devices get DHCP with no problem with DHCP Server Cluster
Symptoms
  • Wireless AP's not getting DHCP addresses, but other devices get DHCP with no problem with DHCP Server Cluster
  • Two Windows Server 2012 setup in load balanced DHCP
Environment
  • Wireless
  • All software versions
  • DHCP Server 2012 cluster mode
  • All Access Points
Cause
  • Microsoft relies on the "secs" value for time in seconds in DHCP Discover packets for the device has been trying to obtain a DHCP address.
  • Our AP's do not increment this "secs" value, so the DHCP server never forwards the request to the secondary server when X amount of seconds is met
  • The DHCP Discover packets typically should hit both servers, and the server will hash out who should respond with an offer based on a MAC address hash algorithm.
  • If the ip-helper is configured on the router for only one of two+ DHCP Servers, some AP's are sending DHCP requests to to the Server may not be going to the correct server.
  • According to Microsoft, even if only one of the IP helpers was configured, after a failure of DHCP for so many seconds, the windows server should have seen in the DHCP discover packet the "secs" value incrementing, and once it hits 6 seconds the server will forward the request over to the other load-balanced DHCP server.
Resolution
  • Verify the IP helper addresses are correctly configured to point to all DHCP servers in a Cluster, not just one or the other.
  • Windows machines, or other devices may not see an issue as they most likely increment the "secs" value during DHCP discovery
Additional notes
  • secs = Filled in by client in DHCP Discover, seconds elapsed since client began address acquisition or renewal process.
  • DHCP RFC: https://www.ietf.org/rfc/rfc2131.txt 
  • secs value is not a requirement of DHCP, but rather optional

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