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DHCP and IP Helper on a Cisco 800 router

Unanswered Question
Aug 13th, 2006
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I have a series of sites with Cisco 800 series routers to connect to head office.

These routers assign DHCP to the small number of clients in each location. We have begun a rollout of desktop management software which can also do PXE booting to image machines in the remote locations, etc..

I have put an IP HELPER-ADDRESS in one of the branch routers, but it seems that does not work (the machine does not PXE boot).

Our carrier seems to think that the client will grab a DHCP address from the router, then not try the ip helper since it already has its address.

Can someone please confirm if that is the case, or if there is something we can do to make this setup work.

Thanks in advance!

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vijayasankar Sun, 08/13/2006 - 21:40
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We need more information regarding your setup to understand the scenario properly.

Does the DHCP server and boot image server located in the same machine or different machine?

If both are different machines, then

Generally for PXE process to work properly, you should be configuring mentioning the ip address of both the DHCP server and the boot image server by using the ip helper-address command. Then it should work.

Here's an outline on PXE boot process in conjunction with DHCP server and Microsoft RIS server.


Hope this helps.


TalonKarrde Sun, 08/13/2006 - 22:58
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Hi VJ,

The DHCP is controlled by our in office router at that site, but the boot image server is located in the head office across a WAN link, which the DHCP router connects to.

We do not want centralized DHCP because if the link goes down then the remote sites will lose connectivity locally.

With that in mind, I dont think that ip helper needs to mention the dhcp server (since DHCP is the same unit which has IP helper).


vijayasankar Sun, 08/13/2006 - 23:17
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Thanks for the update.

If my understanding is correct, every remote site has a local DHCP server on lan.

But your boot image server is located in Central office (separated by a WAN link).

If so, then you have to configure the boot image servers ip address as ip helper-address in branch office router.

Once the DHCP client has received the DHCP ip, then it will send another DHCP discover packet requesting for the boot image server ip, this request will be forwarded to the boot image server at central site and it will reply back.

As explained in the URL mentioned above, the process is as follows,

After the client has an IP address, it must obtain a PXE boot server.

The client sends out another DHCP DISCOVER to locate one:

DHCP DISCOVER from client (client now has an IP address, but is still looking for a PXE boot server)

DHCP OFFER from DHCP server( from dhcp server on local lan, it will re-offer the same IP address)

The same dhcp discover will be forwarded to the central server also.

DHCP OFFER from boot image server (will offer a boot server)

DHCP REQUEST from client to boot image server (requesting the boot server)

DHCP ACK from boot image server ( this will have all the details that the client needs to download the boot image..servername, serverip, file to download etc.)

Hope this helps.


vijayasankar Sun, 08/13/2006 - 23:26
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I think you are mentioning that the DHCP server service is configured in teh branch office router to provide DHCP ip to the clients on that lan.

Still you need to configure the ip helper-address in the branch office router. Refer to the flow mentioned in the above post.



TalonKarrde Wed, 08/16/2006 - 21:44
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Yes, the branch office router is configured with a DHCP scope for the clients in that office.

The router also contains an ip helper-address on the Ethernet interface (where the DHCP clients are located).

Do we need to put something in the DHCP scope instead of on the Ethernet interface, or is there some additional command I would need to put in the router?

Thank you again for all your help.

vijayasankar Wed, 08/16/2006 - 22:26
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For the clients to get DHCP ip, all you have to ensure is to have the proper DHCP server configuration in the router, which you have already done.

It would be nice if you provide the config here.

Inorder to get boot image, the clients will send another DHCP request, requesting for the bootimage server. This request should reach the PXE server that you have in the central office.

So in the router, under the ethernet interface, you should have the ip helper-address configured pointing to the PXE server at the central site.

This should be sufficient for this to work.

Best Regards


jackyoung Wed, 08/16/2006 - 23:27
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If follow VJ's suggestion still not work. This is another issue that you have to ensure the assigned DHCP client IP address can reach the PXE server. Check the routing and ping to the PXE server and PXE server ping to the client too to verify the connectivity.

Hope this helps.

TalonKarrde Thu, 08/17/2006 - 16:05
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Hi VJ,

Thanks again for your help and input (and to everyone who responded).

Indeed, the client and server can ping each other from their respective subnets, so I dont see a routing issue interfering.

One thought that has just occured to me is that our providers network is such that there are routers inbetween our locations in the center of our network (that is the type of network structure we have from our provider). Could these central routers be interfering with the transfer of PXE data?

Also, I have posted a snip of the config from a branch router.

ip dhcp excluded-address


ip dhcp pool LOCAL




lease 0 8


interface Ethernet0

description LAN

ip address

ip helper-address

hold-queue 100 out

It all looks fine, but just does not seem to work (we never get a completion notice from target to indicate that PXE boot worked).

Let me know if there is anything which looks unusual here and I will see if I cant find anything from our carrier as well.

Once more, thank you all for your help with this matter.

jackyoung Thu, 08/17/2006 - 17:25
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I don't find any problem on the configuration and you may need to debug the DHCP packet to observe any finding. e.g. Any information exchange from the PC to the PXE server ?

Hope this helps.

vijayasankar Thu, 08/17/2006 - 20:13
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As stated by Jack Young, there is no problem with the router config.

We have to observe what happens during the process using Debugging.

Here's what you can do parallely during the process, to probe more details on this issue.

1) Run a sniffer/ethereal trace on the client

2) Run a sniffer/ethereal trace on the PXE server.

3) Enable debug in the router to capture the DHCP frames.



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