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DHCP NACK and DHCP Relay for Router RRS feed

  • Question

  • I have recently changed my netconfiguration around and I am have some DHCP trouble

    I have a 3Com 5500G-EI of which i created a vlan and dhcp relay for x.x.200.x subnet (Class C)

    I can get a DHCP address for the clients , However the problem is that sometimes (alot) I get server busy or no ip at all.

    When looking at the dhcp log i noticed that it is being pounded by the switch. so I must have something configured incorrectly and need some help figuring out what.

    may log is full of these......where 0016E0BC7300 is the mac address of the switch

    15,05/04/10,11:54:57,NACK,192.168.200.33,,0016E0BC7300,
    15,05/04/10,11:55:02,NACK,192.168.200.42,,0016E0BC7300,
    15,05/04/10,11:55:07,NACK,192.168.200.24,,0016E0BC7300,
    15,05/04/10,11:55:12,NACK,192.168.200.22,,0016E0BC7300,
    15,05/04/10,11:55:17,NACK,192.168.200.19,,0016E0BC7300,
    15,05/04/10,11:55:22,NACK,192.168.200.15,,0016E0BC7300,
    15,05/04/10,11:55:27,NACK,192.168.200.16,,0016E0BC7300,
    15,05/04/10,11:55:32,NACK,192.168.200.36,,0016E0BC7300,
    15,05/04/10,11:55:37,NACK,192.168.200.35,,0016E0BC7300,
    15,05/04/10,11:55:42,NACK,192.168.200.13,,0016E0BC7300,
    15,05/04/10,11:55:47,NACK,192.168.200.32,,0016E0BC7300,
    15,05/04/10,11:55:52,NACK,192.168.200.30,,0016E0BC7300,
    11,05/04/10,11:55:53,Renew,192.168.200.19,,001EC2DF615E

    I am in need of help.....

    DHCP Server is 2003

    Randy

    Tuesday, May 4, 2010 6:56 PM

Answers

  • Could this be a management port on the switch that is set up as a DHCP client as well?  If so, configure that managed switch with a static IP address.

    This is probably not the best source log for troubleshooting.  It would be better if you took a packet capture and check the GIADDR field in the packet to see if its coming from the remote side.  The relay agent will populate this field when converting from a broadcast to unicast packet.

    The DHCP Relay Process

    When a router for such a subnet receives a DHCP broadcast, it converts it to unicast (with a destination MAC/IP address of the configured DHCP server, source MAC/IP of the router itself). The GIADDR field of this modified request is populated with the IP address of the router interface on which it received the original DHCP request. The DHCP server uses the GIADDR field to identify the subnet of the originating device in order to select an IP address from the correct pool. The DHCP server then sends the DHCP OFFER back to the router via unicast. The router then converts the DHCP OFFER back to a broadcast, sent out on the interface of the original device.


    Visit my blog: anITKB.com, an IT Knowledge Base.
    • Marked as answer by Miles Zhang Friday, May 7, 2010 2:42 AM
    Wednesday, May 5, 2010 2:05 AM

All replies

  • Could this be a management port on the switch that is set up as a DHCP client as well?  If so, configure that managed switch with a static IP address.

    This is probably not the best source log for troubleshooting.  It would be better if you took a packet capture and check the GIADDR field in the packet to see if its coming from the remote side.  The relay agent will populate this field when converting from a broadcast to unicast packet.

    The DHCP Relay Process

    When a router for such a subnet receives a DHCP broadcast, it converts it to unicast (with a destination MAC/IP address of the configured DHCP server, source MAC/IP of the router itself). The GIADDR field of this modified request is populated with the IP address of the router interface on which it received the original DHCP request. The DHCP server uses the GIADDR field to identify the subnet of the originating device in order to select an IP address from the correct pool. The DHCP server then sends the DHCP OFFER back to the router via unicast. The router then converts the DHCP OFFER back to a broadcast, sent out on the interface of the original device.


    Visit my blog: anITKB.com, an IT Knowledge Base.
    • Marked as answer by Miles Zhang Friday, May 7, 2010 2:42 AM
    Wednesday, May 5, 2010 2:05 AM
  • I have the same situation with a CISCO 2921 router as a relay agent. What's the solution?

    Thank you!

    Monday, November 12, 2012 5:32 AM