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DHCP Relay - many SSID with different VLAN

namezys
New Contributor III

Hello. Maybe my understanding of the DHCP relay feature is wrong but let me try to explain what I want to get.

Current setup with general broadcast DHCP

Two SSID:

  • "ssid-10" with VLAN 10
  • "ssid-20" with VLAN 20

Everything is connected through a switch to MikroTik there I have a DHCP server

  • VLAN 10: 10.10.0.1, pool: 10.10.x.x
  • VLAN 20: 10.20.0.1, pool: 10.20.x.x

Everything works as expected:

  • if I connected to "ssid-10", I would get 10.10.x.x
  • if I connected to "ssid-20", I would get 10.20.x.x

DHCP Relay

I hope that I understand correctly how it should work (AP works as relay agent):

  1. wireless STA sends a broadcast DHCP request
  2. AP gets this broadcast request and sends a unicast request to the DHCP server that I preconfigured
    1. In other words, I have to configure different DHCP servers for each SSID: 10.10.0.1 and 10.20.0.1
  3. DHCP server responses to AP
  4. AP sends a response to the STA

Looks like that should work better than broadcast. However, without DHCP relay it works well so I'm just curios if it's possible to setup.

 

 

1 ACCEPTED SOLUTION

No, the DHCP relay is only configured in devices that works in L3. The AP works in L2.
We have the option "Directed MC/BC Threshold" that reduces multicast and broadcast traffic, but it doesn't apply to DHCP and ARP traffic:
https://docs.commscope.com/bundle/unleashed-200.9-onlinehelp/page/GUID-92FB1860-3276-4819-8A0F-78116...

 

Bruno Andrade | Principal TSE Bulldog Americas | RCNA | CWNA | CWDP
Follow me on LinkedIn

View solution in original post

3 REPLIES 3

bruno_andrade
RUCKUS Team Member

Hello,

The DHCP relay is configured in the L3 device of your network.
The DHCP relays is used when your DHCP server is not in the same broadcast domain as your clientes.
E.G: 

SSIDs:

  • VLAN 10: 10.10.0.1, pool: 10.10.x.x
  • VLAN 20: 10.20.0.1, pool: 10.20.x.x

Server DHCP is on:

  • VLAN 30: 10.30.0.200

Since the DHCP server is not in the same broadcast domain as VLANs 10 and 20, you use the DHCP relay functionality on the L3 device (likely a router or layer 3 switch) in each VLAN. This allows the L3 device to forward DHCP requests from clients in VLANs 10 and 20 to the DHCP server in VLAN 30.

The DHCP relay agent typically adds information to the DHCP request packet, such as the original subnet from which the request came. This information helps the DHCP server to assign an IP address from the correct pool for the requesting subnet.

I hope this clarify the DHCP relay usage for you.

 

Bruno Andrade | Principal TSE Bulldog Americas | RCNA | CWNA | CWDP
Follow me on LinkedIn

Yep. I know that the main reason for a DHCP relay is to have a DHCP server in another broadcast domain.
BTW, using the same technology AP can avoid broadcast traffic, can't it? I suppose that option 82 can be used instead of many DHCP servers.

No, the DHCP relay is only configured in devices that works in L3. The AP works in L2.
We have the option "Directed MC/BC Threshold" that reduces multicast and broadcast traffic, but it doesn't apply to DHCP and ARP traffic:
https://docs.commscope.com/bundle/unleashed-200.9-onlinehelp/page/GUID-92FB1860-3276-4819-8A0F-78116...

 

Bruno Andrade | Principal TSE Bulldog Americas | RCNA | CWNA | CWDP
Follow me on LinkedIn