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How to configure DHCP server for multiple subnets with X430-24t switches?


Hi All,

Our department manages its own network in our University. All the network switches (X430-24t/48t) are connected to gateway network switch(Extreme switch only) which is provided by the University.

Each Switch has own its subnet. We want to run a common DHCP server for our department. For this, we have to install a DHCP server on one machine but only machines connected to that subnet/Switch is able to get IP address from DHCP server. On each switch, we are using Bootrelay to forward DHCP request to dedicated DHCP server. Bootrelay configuration is not done at gateway switch as we don't have access to it.

Using nc and tshark, we found DHCP packets are not reaching to DHCP server from any other subnet. Do we need to configure Bootrelay configuration on gateway switch also to work our DHCP setup?

Thanks,
V Arun

10 replies

It's pretty simple, for VLANs theat don't have a DHCP server on them, you're going to need a DHCP relay. Otherwise, you will need a DHCP server on the subnet looking for a request.
Userlevel 2
Hi Arun,

Did you enable ipforwarding on each client vlans? If possible, could you share your configuration?
Hi Guys,

Thanks for fast reply.
Network configurations are as below:
IP address of Gateway switch: 10.119.1.250 Subnet: 255.255.255.0
IP address of switch1: 10.119.11.251 Subnet: 255.255.255.0
IP address of switch2: 10.119.21.251 Subnet: 255.255.255.0
IP address of switch3: 10.119.31.251 Subnet: 255.255.255.0

All switches are connected to gateway switch. A dedicated VLAN is created on each switch subnet on a port of gateway.

DHCP server is on the switch1 subnet. So, machines in switch2 subnet and switch3 subnet has to send DHCP request to DHCP server via gateway server. Bootrelay and IPforwarding are enabled on each switch but not on gateway switch as we don't have access to that switch.

We are observing that we are not receiving DHCP request packet from other subnets than switch1 subnet.

So, I want to know that as we have created VLAN on gateway switch, do we need to enable Bootrelay and IPforawarding accordingly on the gateway switch?

Thanks,
V Arun
Userlevel 6
Arun Verma wrote:

Hi Guys,

Thanks for fast reply.
Network configurations are as below:
IP address of Gateway switch: 10.119.1.250 Subnet: 255.255.255.0
IP address of switch1: 10.119.11.251 Subnet: 255.255.255.0
IP address of switch2: 10.119.21.251 Subnet: 255.255.255.0
IP address of switch3: 10.119.31.251 Subnet: 255.255.255.0

All switches are connected to gateway switch. A dedicated VLAN is created on each switch subnet on a port of gateway.

DHCP server is on the switch1 subnet. So, machines in switch2 subnet and switch3 subnet has to send DHCP request to DHCP server via gateway server. Bootrelay and IPforwarding are enabled on each switch but not on gateway switch as we don't have access to that switch.

We are observing that we are not receiving DHCP request packet from other subnets than switch1 subnet.

So, I want to know that as we have created VLAN on gateway switch, do we need to enable Bootrelay and IPforawarding accordingly on the gateway switch?

Thanks,
V Arun

Hello Arun,

Relay
agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet.

That means you have to enable routing between those subnets + DHCP Relay on the gateway switch (that will be acting as the Relay agent).
Arun Verma wrote:

Hi Guys,

Thanks for fast reply.
Network configurations are as below:
IP address of Gateway switch: 10.119.1.250 Subnet: 255.255.255.0
IP address of switch1: 10.119.11.251 Subnet: 255.255.255.0
IP address of switch2: 10.119.21.251 Subnet: 255.255.255.0
IP address of switch3: 10.119.31.251 Subnet: 255.255.255.0

All switches are connected to gateway switch. A dedicated VLAN is created on each switch subnet on a port of gateway.

DHCP server is on the switch1 subnet. So, machines in switch2 subnet and switch3 subnet has to send DHCP request to DHCP server via gateway server. Bootrelay and IPforwarding are enabled on each switch but not on gateway switch as we don't have access to that switch.

We are observing that we are not receiving DHCP request packet from other subnets than switch1 subnet.

So, I want to know that as we have created VLAN on gateway switch, do we need to enable Bootrelay and IPforawarding accordingly on the gateway switch?

Thanks,
V Arun

Thanks for the reply.
Userlevel 6
Hello Arun,

In order to allow multiple subnets to reach a DHCP server you would need to use what Jeremy recommended which is bootprelay. Below is an article that explains this configuration:

https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-configure-Bootprelay

This should be configured on whatever device is doing the routing. Please keep in mind that the server vlan needs to be added into the bootprelay configuration in order for this to work properly.

Hope this helps!
Userlevel 7
As far as I know the X430 switch is layer 2 only and does not support layer 3 routing nor DHCP relay.
Userlevel 6
Erik Auerswald wrote:

As far as I know the X430 switch is layer 2 only and does not support layer 3 routing nor DHCP relay.

Exactly.
Erik Auerswald wrote:

As far as I know the X430 switch is layer 2 only and does not support layer 3 routing nor DHCP relay.

We are using Summit X440-24p(Layer 3) as gateway switch for routing between subnets.
Userlevel 6
Erik Auerswald wrote:

As far as I know the X430 switch is layer 2 only and does not support layer 3 routing nor DHCP relay.

Hi Arun, if X440 is the gateway switch, could you please enable and configure DHCP relay to that X440 and let us know if that worked?

As already shared before, the relay must be done by the switch that's routing among those subnets.
Erik Auerswald wrote:

As far as I know the X430 switch is layer 2 only and does not support layer 3 routing nor DHCP relay.

Ya, Sure. Thanks for quick reply.

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