Configuring local DHCP database in conjunction with IP helper

[jorge75medina]

Real quick question, If I have a router, that is configured to utilize DHCP via ip helper address to distribute ip addresses from an external server, and I configure the router with a local dhcp configuration, will it revert to the local database and negate the external dhcp server configuration? Please help. :

[luis_cordova]

Hi @jorge75medina 

As far as I know, then you will have 2 DHCP servers, local and external.
The PC will get addressing from the DHCP server that answers first.
The problem is that there could be IP address conflicts if both DHCP servers are giving the same segment of IP addresses.

Regards

[jorge75medina]

Makes sense, yes it will have local and external, but the new local DHCP configuration will be for future ip address space and local usage of the new space vice an external dhcp server. It will be a completely different address space.

[luis_cordova]

Hi @jorge75medina 

If the IP segments provided by the servers is different, then you will not have an IP address conflict.

Now, if the 2 DHCP pools (1 per DHCP server) have the same ip gateway, then both servers (local and external) will be enabled to provide addresses to your PCs, so the PCs will get addressing from the server that responds first and it will adopt an address from that server.

If the pools of both servers provide IP addresses of different networks, then the PCs will obtain addresses from the corresponding pool, either from the local server or the external server.

Regards

[jorge75medina]

Gotcha again. :) They further clarify, they will have different gateways since they it will be new address space. However the gateways for the new local DHCP server have not been configured yet so it will be pointing to a non-existent gateway. Bottom line, it sound like configuring a local dhcp server will not cause any issues if it is just the DHCP config with no active interfaces on the new dhcp scopes. 

[luis_cordova]

Hi @jorge75medina 

Right.
DHCP servers (local or external) assign addressing using the network's ip gateway entered into each pool.
If the gateway address indicated in a pool is not yet configured or the interface where it is configured is down, then the server will not deliver addresses from that pool.

Regards

[Meheretab Mengistu]

One thing you can do is exclude all IP addresses from the local DHCP Pool and it can not issue any IP address from the local pool. Once you are ready to use the local pool, remove the excluded-address statement for the range you want to use.

For example, if your pool is 172.16.0.0/20, you can add:

ip dhcp excluded-addresses 172.16.0.1 172.16.0.255
ip dhcp excluded-addresses 172.16.1.0 172.16.14.255
ip dhcp excluded-addresses 172.16.15.1 172.16.15.255

And when you are ready to use, you can remove exclusion of the range you want to use. Note that: I divided the exclusion list to make it easier to remove when you are ready to put it back to the pool.

no ip address excluded-address 172.16.1.0 172.16.14.255

HTH,
Meheretab

[Scott Leport]

Your hosts will pick whatever DHCP server it can contact first.

What are your requirements? Is it curiosity or do you have a DHCP migration coming up?

[paul driver]

Hello
As stated if you have two dhcp servers the client will receive an allocation from whichever server responds first however there is command that when applied will tell the dhcp relay to seek the same dhcp server that a client had previously used for allocation Ip dhcp relay prefer known-good-server

Also it is best practice to split the active scopes over the two servers, This will negate conflicts and provide resiliency for your clients if once of the servers fail..