DHCP Server assigning address to multiple VLANs on multiple layer 3 switches

[asteep1@lsu.edu]

Hello all,

For my Communications in Computing course, I am trying to figure out how to get my DHCP server to be able to assign IPs to two different VLANs (both VLANs are present on both switches). The DHCP functionality works fine until I assign the interfaces to a particular VLAN then the DHCP fails. I have tried many different things but nothing works. Attached is an image of the network. 

[luis_cordova]

Hi asteep1@lsu.edu ,

I suggest you compress your exercise (using Winzip, for example) and attach it to see your settings.

For the meantime, I'll give you an example:

If you configure an SVI on each L3 switch, you are creating a broadcast domain for each vlan for each switch.
That is, if you have vlan 10 and 20 distributed on each switch, you will have 4 broadcast domains.
Each broadcast domain requires a different DHCP pool.

Now, if you leave only the SVI of VLAN 10 and 20 in a single L3 switch and the other switches communicate with it through trunk links, you will only have 2 broadcast domains and, therefore, you will only need 2 DHCP pools.

As mentioned before, if the DHCP server is part of a vlan, then the DHCP requests will reach the server through broadcast.
VLANs that do not have a DHCP server inside them, must have configured in their SVI the ip helper-address command, pointing to the DHCP server.

Regards

[Seb Rupik]

Hi there,

On each multi-layer switch, on the interface which routes the client traffic configure a DHCP relay:

!
int vlan100
  ip helper-address <IP_OF_DHCP_SERVER>
!

Make sure that the DHCP has the correct pools configured for the client subnets.

cheers,

Seb.

[Jaderson Pessoa]

Hi there

On SWITCH L3 that has VLAN with ip address configure under this interface " ip helper-address IP OF DHCP"

If you can input your .PT file here, you can help your better;

Thanks in advance.