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DHCP-Server padding

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  • 1.  DHCP-Server padding

    Posted Dec 20, 2019 06:25 AM

    Hi community,

     

    I hope I choose the right category (my first post).

    Here is my problem:

    I run a DHCP server on a  Aruba 3810 M switch. It works for almost every client. But I have a small server that can't get an IP address when using dhcp. I took a sniffer trace and found that the server sets DHCP option 57 (Maximum DHCP Message Size) to 576 in the DHCP Discover message. The switch is answering the request with and Offer but it is padding the answer and creates a 1518 bytes packet. It looks like the client does not accept the Offer and keeps sending Discover messages.

     

    I traced what happens when a WIN10 computer send a Discover. The WIN10 client does not set option 57 and the switchs replies with an Offer that is 300+ bytes long. No padding to 1518 at all.

     

    DHCP Server config:

     

    dhcp-server pool "pool-Office-11"
    authoritative
    default-router "x.y.11.1"
    dns-server "x.y.0.10,x.z.0.11,1.1.1.1"
    network x.y.11.0 255.255.255.0
    range x.y.11.100 x.y.11.199

     

    Does anybody have an idea to prevent padding to 1518 bytes on the aruba switch?



  • 2.  RE: DHCP-Server padding

    Posted Dec 20, 2019 08:39 AM
      |   view attached

    Here the Discover and Offer packets.

     

    What I didn't metion, DHCP works fine for the server in other environments.

    Attachment(s)

    txt
    dhcp_usbserver.txt   3 KB 1 version


  • 3.  RE: DHCP-Server padding

    MVP GURU
    Posted Dec 20, 2019 04:07 PM

    What firmware version ?

     

    may be need to use relay DHCP ?



  • 4.  RE: DHCP-Server padding

    Posted Dec 21, 2019 02:24 AM

    Hi,

     

    software version is

     

    Software revision : KB.16.07.0003

     

     

    The Swtich/DHCP Server has the vlan in doubt directly connected so there is no need for a relay.

     

     

     

     

     



  • 5.  RE: DHCP-Server padding

    MVP GURU
    Posted Dec 23, 2019 03:49 AM
    @jobearuba wrote:

    Hi,

     

    software version is

     

    Software revision : KB.16.07.0003

     

     

    The Swtich/DHCP Server has the vlan in doubt directly connected so there is no need for a relay.

     

     

    Hi,

    I should be a good idea to upgrade the firmware

     

    About DHCP relay, the idea is to use a another DHCP server



  • 6.  RE: DHCP-Server padding

    Posted Jan 02, 2020 10:17 AM

    Hi everyone,

     

    Happy New Year!

     

    Thank you for the replies. I did some SW upgrades using the latest realeased version in the respective train. I try to stay as low in the major version as I could.

     

    WC.16.07.0006 (July 2019):

    No change in DHCP behaviour. Still all DHCP packet send by the switch are maximum size for the medium (1514 Ethernet, 1518 with dot1q header)

     

    WC.16.08.0008 (November 2019)

    Padding for DHCP packets is gone. Packets are just the size they need to be (3xx bytes). That looks promising but the success needs to be verified at the production network.

     

    I couldn't find any release note mentioning a change in DHCP.

    I'll keep you posted.