topic Re: Multi hop DHCP relay in General Topics

Multi hop DHCP relay

Alex_Samad — Mon, 19 Feb 2018 23:23:41 GMT

So I want to get my VOIP phones to dhcp to the vPBX.

Phone are on a vlan in the office vPBX is in the DC

so vlan for phone -> PA -> vlan -> arista switch -> vlan -> PA (clustered A/A) -> vlan -> vPBX

So I can setup DHCP relay on the first PA and I can set the DHCP server as being the ip of the vPBX, and I believe as it will be a unicast and not broadcast it should make it there.

But I am think of the response how does it make it back as it will be a broadcast response ...

can I do it ?

Re: Multi hop DHCP relay

Remo — Mon, 19 Feb 2018 23:44:52 GMT

Hi @Alex_Samad

On the first PA you need to configure a DHCP relay agent (https://www.paloaltonetworks.com/documentation/80/pan-os/pan-os/networking/dhcp/configure-an-interface-as-a-dhcp-relay-agent)

This DHCP relay agent keeps track of the received DHCP requests to that rhe unicast reply from the DHCP server will find its way back to the device which sent the broadcast request.

Re: Multi hop DHCP relay

Alex_Samad — Mon, 19 Feb 2018 23:48:04 GMT

are you suggesting that the dhcp server will reply via unicast so that the PA which doesn't have dhcp relay agent on it will know how to route it back ?

so my understanding is

device -> dhpc request (broadcast) -> PA (with agent) -> forwards via unicast -> router (arista) -> router (arisa) -> PA -> dhcp server.

but the path back is

dhcp server -> broadcast ... this doesn't go any where as the PA doesn't know about it, I think

Re: Multi hop DHCP relay

Remo — Mon, 19 Feb 2018 23:57:19 GMT

The way back is only a unicast which the PA (with DHCP relay agent) then forwards to the right device based on the mac address

Re: Multi hop DHCP relay

Alex_Samad — Tue, 20 Feb 2018 00:01:38 GMT

okay, I will give it a go

Re: Multi hop DHCP relay

OtakarKlier — Tue, 20 Feb 2018 17:04:07 GMT

Hello,

As @Remo state is will work. I have a similar setup and all you need is the DHCP relay setup on the PAN that is performing the relay. The reply addresss becomes the PAN's IP and hte PAN knows where to send it based on MAC address.

Also make sure to allow the DHCP applications/ports on both PANs.

Regards,