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Apologies for my newbie question. I have recently had a Mellanox switch added to the mix. I was using an "IP helper-address" to forward DHCP requests in different VLANs to my DHCP server(s) on my other switches. I see you can configure a DHCP relay agent on the Mellanox switch but it was my understanding that this works differently from the IP helper-address. i.e in the DHCP relay the router receives the request and builds a new request to send to the server. I see there is a vrf-auto-helper but I can't quite get my head around what this is supposed to do.

On a Cisco-esque switch I have:

interface VLAN ??
 ip address a.b.c.254 255.255.255.0
 ip helper-address {address of dhcp server}

How would I achieve the same result on a Mellanox switch?

Would this be an equivalent?

# DHCP relay configuration
##
   ip dhcp relay instance 1 vrf default
   ip dhcp relay instance 1 vrf-auto-helper
   ip dhcp relay instance 1 address {address of dhcp server}
   ip dhcp relay instance 1 always-on
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2 Answers 2

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Here is a configuration example on Mellanox Onyx:

##
## DHCP relay configuration
##
   ip dhcp relay instance 1 vrf default
   ip dhcp relay instance 1 address 10.100.100.1
   ip dhcp relay instance 1 always-on
   interface vlan 1 ip dhcp relay instance 1 downstream
   interface vlan 30 ip dhcp relay instance 1 downstream
   interface vlan 197 ip dhcp relay instance 1 downstream

As you can see the configuration is not done under the vlan interface, but rather the VLAN(s) are added in the dhcp instance relay configuration.

Edit after question update:

Yes using:

 ip dhcp relay instance 1 vrf-auto-helper

should enable the configured dhcp relay for all L3 interfaces within the specified VRF

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I have a similar problem understanding DHCP relay in ONYX. We have VRFs for each network segment. As we have more then 20 segments, the amount is bigger then the max amount of relay instance (8) that is allowed on onyx. Each VRF has 3 VLAN and IP interfaces. Two of them are downstream and one is upstream. How can i achive to have relay active on all VRFs??

I tried assigning the relay instance to several VRF, but it seems that one instance can only be assined to one VRF.

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  • It might be worth not to un-earth an old closed/answered question/thread, but to open ne new one, and to including a cross-reference/link to this one. Aug 25, 2023 at 10:27
  • That seems like you have reached a design limit of the equipment. If you cannot have an unlimited number of DHCP relay instances to match the number of VRFs, then perhaps you need to have fewer VRFs or simply use more DHCP server instances or connect your DHCP server to the various networks directly. Aug 25, 2023 at 14:39

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