DHCP hands out an IP address but not DHCP Options like 78 and 79

  • 7009839
  • 02-Dec-2011
  • 30-Apr-2012

Environment

DHCP

Situation

With a DHCP server set up and configured to hand out additional DHCP options, workstations get an IP address but they do not get the additionally configured DHCP options.

Taking simultaneous LAN traces shows that there is an IP Relay involved.  The DHCP DISCOVER, DHCP OFFER, DHCP REQUEST, and DHCP ACK are all relayed accordingly to the DHCP server and then through the IP Relay agent back to the host.

Now that the host has a DHCP provided IP address, an application on the host can get additional information from the DHCP server by way of the DHCP INFORM mechanism.  The host sends out a DHCP INFORM packet.  The IP relay agent forwards it on to the DHCP server.  The DHCP server sends a DHCP ACK packet with the requested information back to the IP Relay, as it has for all other DHCP replies.  The host never gets that DHCP ACK with the requested DHCP Options.

Resolution

This situation has been shown to be a configuration issue on the IP Relay Agent.

The feature that is enabled that causes this issue is a security feature on the third party switch with a name like, DHCP Snooping.

Additional Information

A wiki link describing DHCP Snooping:
http://en.wikipedia.org/wiki/DHCP_snooping


A Cisco document for the Catalyst 6500 series of switches and DHCP Snooping:
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/snoodhcp.html

Feedback service temporarily unavailable. For content questions or problems, please contact Support.