topic Re: DHCP Relay Issue in Switching

DHCP Relay Issue

OddievenInc — Sat, 10 Aug 2024 13:05:30 GMT

Hi everyone,

I'm having issues with obtaining DHCP from a different subnet. Did some troubleshooting, but got to the point where I had to reach out for support.

I'm trying to obtain a DHCP IP for the client directly connected to the Branch Office Core switch. I can ping the DHCP Server IP from the branch office core switch.

Attached is the topology, and configuration related to the DHCP relay etc. Is this viable? If it's wrong what is the correct way of setting things up? Appreciate your time and comments.

Re: DHCP Relay Issue

Mark Elsen — Sat, 10 Aug 2024 13:40:23 GMT

- Diagram is a bit overwhelming , on a first reply ; did you define a dhcp relay statement in the vlan configuration for clients to be able to find a dhcp server when using that particular vlan ?

Re: DHCP Relay Issue

OddievenInc — Sat, 10 Aug 2024 13:43:39 GMT

HI thanks for the response. As you can see, have configured ip helper-address under the interface vlan 10 on the branch office core switch. The client connecting to the switchport is, on access vlan 10

Re: DHCP Relay Issue

Mark Elsen — Sat, 10 Aug 2024 14:07:30 GMT

- Not sure how far troubleshooting was done so far ; concerning the DHCP server ,
on IOS you have debug dhcp detail for instance and
debug ip dhcp server events
Look for similar commands on your dhcp server platform (if applicable)

+ I notice that this is a branch office , make sure no corporate dmz-firewalling blocks full dhcp services
towards the branch office ,

You may also find this useful : https://networklessons.com/cisco/ccie-routing-switching-written/troubleshooting-dhcp

Re: DHCP Relay Issue

OddievenInc — Sat, 10 Aug 2024 14:11:04 GMT

Thanks again for the reply. Appreciate it. I have done debugs & taken monitor session captures. Also, there is no firewall in the middle. ISP has allowed all communication. ISP link is L2, but local termination is L3 (as you can see in the topology) Do you think this should work? Or is there something that i have to look at in terms of the design?

Re: DHCP Relay Issue

Mark Elsen — Sat, 10 Aug 2024 14:18:57 GMT

>..., but local termination is L3
- This might be an issue because ISP's normally expect ip traffic and dhcp is layer2 , you may need to contact the ISP
and check if what you want can be done, In the mean time you could deploy a dhcp server on the branch office
(temporarily) .

Re: DHCP Relay Issue

Martin L — Sat, 10 Aug 2024 14:27:07 GMT

PC connected to switchport in vlan 10 ? there is no DHCP for network 10, I think I see one DHCP pool for vlan 30 or 36;

Try connecting PC to switchport on vlan 30 or 36; Or add another pool for vlan 10 network

Then, every L3 port (switchport and interface SVI port) needs ip helper address x.x.x.x

Regards, ML
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Re: DHCP Relay Issue

OddievenInc — Sat, 10 Aug 2024 14:28:22 GMT

Hi thank you for the reply. Noted. The things here, is that I have clients in the Head Office configured under vlan30 to receive DHCP IPs, (locally) from the DHCP pool vlan30. I want to extend that same DHCP pool for the Branch Office users in vlan10. Is this even feasible? Because I can only have the SVI for vlan30 (DHCP Pool vlan30) configured in the Head Office core switch.

Re: DHCP Relay Issue

Mark Elsen — Sat, 10 Aug 2024 15:01:23 GMT

- I believe you can always create another SVI too

Re: DHCP Relay Issue

MHM Cisco World — Sat, 10 Aug 2024 15:43:34 GMT

you topolgy is nice but some point not clear
where is dhcp server?

where client connect to?

MHM

Re: DHCP Relay Issue

Harold Ritter — Sat, 10 Aug 2024 16:36:01 GMT

Hi @OddievenInc ,

The host in the branch should get an address from the subnet it is connected to (10.140.48.0/22). Trying to get an address from a pool from another subnet will not work.

You should define a pool locally on the branch office switch.

ip dhcp pool vlan10

network 10.140.48.0 255.255.252.0

default-router 10.140.48.1

dns-server 8.8.8.8

interface vlan10

no ip helper-address 10.140.36.1

Regards,

Re: DHCP Relay Issue

Martin L — Sat, 10 Aug 2024 16:23:34 GMT

"extend vlan 30 DHCP pool for the Branch Office users in vlan10." - I do not think that is possible. I am not aware of any trick that would make this; Every vlan has its own pool with its own subnet; Vlan 10 is under subnet 10.140.48.0 interface vlan 10; you should have separate pool for vlan 10.

Regards, ML
**Please Rate All Helpful Responses **

Re: DHCP Relay Issue

OddievenInc — Sat, 10 Aug 2024 16:49:28 GMT

Hi Harold, Good day.

Thank you ever so much for the response. This is well Noted.

However, I made some changes, would this work? I want to have the DHCP Pool for the Branch Office (vlan 10) configured in the Head Office Core Switch, rather than having it locally. (this is actually due to a future change; where there will be a centralized DHCP server located in the Head Office premises, for both head office and Branch Office.)

I have configured 'interface vlan 10' in both branch office and head office. Pls refer below. Will the branch office send the DISCOVER Packet out of it's exit interface? and reach the Head Office core switch where the DHCP Pool is located, and ultimately client connected to the branch office in vlan 10 access port getting an IP address.

Re: DHCP Relay Issue

OddievenInc — Sat, 10 Aug 2024 16:50:47 GMT

Thank you for the reply. Appreciate it.

I have made some changes. Pls refer below. Would this suffice?

Re: DHCP Relay Issue

OddievenInc — Sat, 10 Aug 2024 16:51:53 GMT

Made some changes. Pls refer below.

Re: DHCP Relay Issue

MHM Cisco World — Sat, 10 Aug 2024 17:20:34 GMT

two same VLAN in two two sites and using ip helper using same subnet,

that not work
you have ISP-ISP L2 but the site stack SW to ISP is L3 ??? can you more elaborate

MHM

Re: DHCP Relay Issue

Harold Ritter — Sat, 10 Aug 2024 17:50:11 GMT

Hi @OddievenInc ,

Do not configure vlan10 on the head office side. Just the pool. You also need to add the following command on the head office side:

ip dhcp excluded-address 10.140.48.1
ip dhcp pool vlan10
network 10.140.48.0 255.255.252.0
default-router 10.140.48.1
dns-server 8.8.8.8

no interface vlan10

and on the branch side:

interface vlan10

ip address 10.140.48.1 255.255.252.0

ip helper-address 10.3.0.1

Regards,

Re: DHCP Relay Issue

OddievenInc — Sat, 10 Aug 2024 17:57:19 GMT

Hi @Harold Ritter

Thank you so much. This is well understood.

I will follow the above steps.

Also, any references/configuration guides to solidify the statement "Do not configure vlan10 on the head office side" I would greatly appreciate if you could provide any if you have please. Would be handy to know the reasoning.

Re: DHCP Relay Issue

Harold Ritter — Sat, 10 Aug 2024 18:40:42 GMT

Hi @OddievenInc ,

Configuring interface vlan10 (same subnet) on both the head office and the branch office does not make sense and would break your routing scheme. The head office router would consider subnet 10.140.48.0/22 to be directly connected and would therefore never send traffic to the host connected to that same subnet in the branch office.

Regards,