topic Re: Secondary IP and DHCP in General Topics

Secondary IP and DHCP

mrsold — Tue, 10 Feb 2015 19:56:52 GMT

Greetings,

Say we have an interface that is configred the following way:

e1/2.240

Tag: 240

IPv4 Address (two addresses on the interface):

-10.10.100.1/24

-192.168.100.1/24

Now, if that interface is configured as a DHCP relay, which network is it going to send in the relay request?

Re: Secondary IP and DHCP

bat — Tue, 10 Feb 2015 21:44:57 GMT

mrsold

Could you please explain your query in more detail ?

I think DHCP requests will be accepted for both networks and will be forwarded to DHCP server ?

Re: Secondary IP and DHCP

mrsold — Tue, 10 Feb 2015 21:55:20 GMT

bat wrote:

mrsold

Could you please explain your query in more detail ?

I think DHCP requests will be accepted for both networks and will be forwarded to DHCP server ?

If I have both of those IPs on the interface, and I have scopes built for both of those networks, does it relay to both of those subnets? Wait until one is exhausted and there is no response?

Re: Secondary IP and DHCP

DavePalo — Wed, 11 Feb 2015 10:34:20 GMT

Hi mrsold

It will only relay for one subnet.

The interface address used to relay the DHCP request (and therefore determine which DHCP scope will be used) is the IP address at the top of the list on your IPv4 tab.

You can adjust the positioning of IP addresses on this tab using the move up/down options should you wish.

Regards,

Dave

Re: Secondary IP and DHCP

mrsold — Wed, 11 Feb 2015 13:25:59 GMT

dyoung wrote:

Hi mrsold

It will only relay for one subnet.

The interface address used to relay the DHCP request (and therefore determine which DHCP scope will be used) is the IP address at the top of the list on your IPv4 tab.

You can adjust the positioning of IP addresses on this tab using the move up/down options should you wish.

Regards,

Dave

Thanks dyoung - do you know if this is documented anywhere?

Re: Secondary IP and DHCP

DavePalo — Wed, 11 Feb 2015 13:39:20 GMT

Hi mrsold, no I've not seen this officially documented anywhere.

Re: Secondary IP and DHCP

indup089 — Thu, 19 Feb 2015 13:40:35 GMT

Hi mrsold,

we can confirm the behaviour described from dyoung, we have PA-7050 as a core-router for several vlans with also dhcp-relay and secondary-IP in use.

The key to solve this issue is that the DHCP server (Bluecat IPAM here) must be able to handle two scopes (the primary IP and seconary) when DHCP requests are received from one IP (in that case the primary IP from the interface-config in the PA used for DHCP relay)

Best,

Ulrich

Re: Secondary IP and DHCP

Unibw — Thu, 19 Feb 2015 21:55:23 GMT

Hi mrsold,

if you happen to use ISC dhcpd look for the shared-network statement use to join together subnets sharing the same L2 segment. ISC dhcpd is actually the motor for most IPAM appliances like Bluecat, Infoblox, ...

Best regards

Winfried

Re: Secondary IP and DHCP

MZenarosa — Wed, 19 Jan 2022 01:42:01 GMT

old thread but may help someone that runs into this issue that works in a M$ shop..

use Super Scopes in M$ DHCP

Re: Secondary IP and DHCP

Pa-Secure — Wed, 17 Jan 2024 13:11:18 GMT

The DHCP relay agent to use secondary IP addresses when the DHCP server does not reply the DHCP-OFFER message. The relay agent initially encapsulates its primary IP address to the giaddr field before forwarding a request to the DHCP server.

But I Did not find any option to enable the smart relay option in PA, whereas I tried with Fortigate it is working