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Additional DHCP IP helper for same vlan

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  • 1.  Additional DHCP IP helper for same vlan

    Posted Jul 01, 2014 01:05 PM

    Hi,

     

    Wi-Fi users are getting IP from DHCP server through controller.

    Can we  add second IP helper for additional DHCP server in controller?  This require for fallback plan of DHCP server.

     

    Kindlly suggest...

     

    Thank You...



  • 2.  RE: Additional DHCP IP helper for same vlan

    EMPLOYEE
    Posted Jul 01, 2014 01:07 PM
    Yes. Use the "ip helper-address" command under the VLAN interface.


  • 3.  RE: Additional DHCP IP helper for same vlan

    Posted Jul 01, 2014 01:19 PM

    Thanks for quick reply..

     

    1) if i add 2 DHCP ip helper for vlan interface then will both DHCP lease IP to user or only first DHCP will lease IP to user.

    2) And if my first DHCP helper down then will second DHCP lease IP to user?



  • 4.  RE: Additional DHCP IP helper for same vlan
    Best Answer

    Posted Jul 02, 2014 04:46 AM

    1) Yes. We can add second IP helper as DHCP server.

    2) If have 2 IP helper addresses configured under vlan interface at the same time, DHCP broadcast will be relayed to both the DHCP servers. Client will accept the DHCP OFFER from the server which responds first.

    3) As both the DHCP servers are using the same IP address range, they will not be aware of the IP addresses leased out from each other. In case one of the servers leases out an IP address which is already leased out from the 2nd DHCP server to a different client, it becomes clients responsibility to send a gratuitous ARP to ensure that IP address assigned to it is not being used.

    4) If by any chance, gratuitous ARP is unable to update the ARP tables for other clients, there could be IP address conflicts on the network.

    5) I will suggest to have a split scope on the DHCP servers as it is difficult to solely rely on client. Otherwise, configure the 2nd IP helper address once the 1st server goes down.