Using External DHCP for multiple VLANs on 3750

[MAK6]

Hi,

I am using catalyst 3750 as a core switch with multiple VLANs and local DHCP Scopes for all the vlans. We will migrate the DHCP to external DHCP server ( Microsoft Server ).

I want to know how external DHCP server can serve the DHCP request from multiple VLANs and assign the correct scope based on the DCHP request from specific VLAN ?

[npress]

Use DHCP Relays on the switch

[Muhammad Awais Khan · Accepted solution]

if the DHCP server and the DHCP clients are on different networks or subnets, you must configure the switch with the ip helper-address address interface configuration command under the VLAN SVI.

The address used in the ip helper-address command can be a specific DHCP server IP address, or it can be the network address if other DHCP servers are on the destination network segment. Using the network address enables any DHCP server to respond to requests.

Config Sample:

Interface vlan x
ip address x.x.x.x x.x.x.x
ip helper-address y.y.y.y ( this is the IP Address for your DHCP Server )

configure above for all the VLANS you are having.

Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_55_se/configuration/guide/3750xscg/swdhcp82.html

[MAK6]

Thank you Muhammad, I will test on some vlans and will share the results.

[Muhammad Awais Khan]

make sure that the SVI's on the switch should be able to reach DHCP Server.

[MAK6]

Thank you for the brief explanation and command reference! It worked for me :)

[luis_cordova]

Hi @MAK6 

I want to know how external DHCP server can serve the DHCP request from multiple VLANs and assign the correct scope based on the DCHP request from specific VLAN ?

DHCP request packets are sent with broadcast destination address and propagated throughout the broadcast domain.
If a DHCP server is located within the broadcast domain, it delivers IP addresses belonging to the scope of that domain.

If the DHCP request packet of a device does not find a DHCP server within the broadcast domain, the timeout is terminated and the operating system of that device assigns it a link-local address.
Now, if the ip helper-address command has been entered in the corresponding interface on a device that serves as a gateway for that domain, then that device replaces the broadcast destination address with a unicast destination address, with the gateway address of that domain as address of origin.

With that packet having a unicast destination address, it can be routed to the DHCP server located on a remote network.

When that packet arrives at the DHCP server, it assigns addresses of the scope corresponding to the network to which the source address of that packet belongs.

In this way, the DHCP server can assign the correct addresses, coming from different vlan.

Sorry if I get too long in the explanation.

Regards

[MAK6]

Thank you for the explaination!!