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Author Topic: DHCP Relay Server as Failover  (Read 6025 times)

John Kizmo

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DHCP Relay Server as Failover
« on: August 25, 2014, 08:46:00 PM »

Hi, I want to add a second server in the DHCP relay configuration but i want only to use this when the server1 is not available. I was able to add a second server but what happen is that relay is happening by load balancing i think since some users are getting there dhcp in the second server. What other settings should i need to configure in my DGS-3620-28TC.

Thank you very much.
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FurryNutz

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Re: DHCP Relay Server as Failover
« Reply #1 on: August 26, 2014, 09:18:49 AM »

Any status on this?

Any information in the User Manual regarding this?

I recommend that you phone contact your regional D-Link support office and ask for help and information regarding this. We find that phone contact has better immediate results over using email.
Let us know how it goes please.
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"Nothing Funny about It...." We are not here to Impress anyone! You have a be a COMPETENT user first to under stand COMPETENT help!

PacketTracer

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Re: DHCP Relay Server as Failover
« Reply #2 on: August 26, 2014, 01:45:43 PM »

Hi John,

what you observe is the standard compliant behaviour of a DHCP relay agent: It always relays any DHCP client packets to all DHCP servers whose IP addresses are entered into the relay configuration. And you can't change this behaviour via configuration. For details see the first DISCUSSION at page 16 in RFC1542 and of course the DHCPv4 basics as described in RFC2131.

The only choice you have, is to operate both DHCP servers in parallel and split the DHCP address pool into two non-overlapping halves, where the first DHCP server uses the first half of the pool and the second DHCP server uses the second half respectively. Of course this only works if the number of DHCP clients inside the LAN or VLAN, the split pool is used for, is smaller than half of the pool such that in case of a failure of one of the DHCP servers all DHCP clients can still get an IP address out of the residual half pool managed by the other still working DHCP server.

PT
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John Kizmo

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Re: DHCP Relay Server as Failover
« Reply #3 on: August 27, 2014, 08:23:26 PM »

Hi,

Thanks for the reply. But can you point me to manual to how can i configure the "split the DHCP address pool"?

I have additional question, in the dhcp relay interfaces settings whenever i add my second server it will always be put on Server1 table, refer to the table below:

Interface Name --- Server1 ------ Server2
Vlan05 ----------- 192.168.20.1 -- 192.168.20.2
Vlan10 ----------- 192.168.20.1 -- 192.168.20.2

I want 192.168.20.2 to be in the server1 but no matter if I add it first or last it will always be put on the server2. Is there any other configuration that i needed to do or what is the trigger why it will always put on the server2 table?

Thank you very much.
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PacketTracer

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Re: DHCP Relay Server as Failover
« Reply #4 on: August 28, 2014, 01:20:18 PM »

Hi John,

Quote
But can you point me to manual to how can i configure the "split the DHCP address pool"?

You won't find anything concerning this topic inside you switch manual, because it is a matter of the configuration of your DHCP servers, not your switch!

For example: Given you have less than 100 DHCP clients inside VLAN 5 and your address pool for these DHCP clients is 192.168.5.20 - 192.168.5.219 (exactly 200 addresses = twice the maximum number of DHCP clients), where 192.168.5.0/24 is the IP network used for VLAN 5, you have the choice to configure either

  • one DHCP server (e.g. server1=192.168.20.1) to deploy addresses out of the pool 192.168.5.20 - 192.168.5.219
  • or two DHCP servers (e.g. server1=192.168.20.1 + server2=192.168.20.2) in parallel, where server1 deploys addresses out of the pool 192.168.5.20 - 192.168.5.119 and server2 deploys addresses out of the pool 192.168.5.120 - 192.168.5.219

If DHCP server1 fails, DHCP clients inside VLAN 5 get addresses from the pool 192.168.5.120 - 192.168.5.219 managed by server2. Vice versa, if DHCP server2 fails, DHCP clients inside VLAN 5 get addresses from the pool 192.168.5.20 - 192.168.5.119 managed by server1.

If server1 and server2 are both active, a DHCP client can get an IP address either from pool 192.168.5.20 - 192.168.5.119 (server1) or 192.168.5.120 - 192.168.5.219 (server2) depending on which server offering the client prefers (usually the one first received, when the client asks for an address the very first time, later it will try to get the same address again by sending a request to the server it got its address from the last time).

Quote
I have additional question, ...

... which I can't answer, because I haven't any experience with D-Link switches.

Just a workaround:
Configure your DHCP server1 using the IP address of server2 (192.168.20.2)
Configure your DHCP server2 using the IP address of server1 (192.168.20.1)

PT

« Last Edit: August 29, 2014, 01:45:07 AM by PacketTracer »
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