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DHCP Clients sending DHCPDECLINE packets

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TitleDHCP Clients sending DHCPDECLINE packets
Symptoms
  • DHCP Clients receive IP addresses from DHCP server (DORA)
  • After new IP is received, DHCP client reject the IP by sending a DHCPDECLINE packet
  • DHCP clients try to obtain an IP address again, then send a DHCPDECLINE and so on.
Environment
  • EXOS ALL
Cause
A firewall on the network was configured for proxy ARP for the VLAN where the DHCP clients are located. Once the DHCP client received an IP address, it sends a gratuitous ARP. The firewall responded with an ARP back to the DHCP client declaring it is a duplicate. That triggered the DHCP client to send a DHCPDECLINE and request a new IP. 
Resolution
  • Ensure Proxy ARP feature is not configured on firewall.
Additional notes
  • A packet capture from the client side will help in determining the sequence of events and packets. 
  • Enabling IP DAD in EXOS can help us in identifying what the source MAC address of the "duplicate" IP is.

Related Articles:
Troubleshooting DHCP issues

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