DHCP pool AND helper-address for PXE?

[jp.briggs]

In our environment, we have DHCP pools that sit on the access L3 switches. We're trying to add the ability to PXE boot to a Windows Deployment server for workstation imaging. From documentation, M$ frowns on the DHCP option route, so we're trying to use ip helper-addresses.

So: Can we have the switch reply with DHCP leases (IP, DNS, Def GW, etc), but also have the SVI configured with ip helper-address <IP of MDT server> to get the PXE boot information? Or is the switch responding to the DHCP request with its dhcp pool, and not forwarding the request on?

Right now it appears that the server is not seeing any DHCP requests from this other VLAN.

More info:
Users -> VLAN100 10.5.206.0/24 with a dhcp pool and an ip helper-address pointing to 10.5.207.245

MDT Server -> VLAN105 10.5.207.240/29

Test workstation plugged into VLAN100 on the same switch as the server, plugged into VLAN105 (all one switch)

[Prabath Godevithanage]

Hi,

Sorry I can't get my head around what device issues your DHCP now,whether it is still your Switch or the Deployment Server. make sure you have required ports opened on the deployment server and antivirus isn't blocking any required ports.I would just try the process in one vlan to make sure that deployment server is functioning as it should.

We recently did this exercise and you may have figured you have two options,"ip-helper" and "dhcp options" things to consider are as below

USING IP HELPER 

If you are to use single IP helper on your SVIs,you'll have to have dhcp components in the deployment server as well.if you have separate DHCP Server and Deployment server and still want to use helper statement instead of DHCP options, I believe you may need to have two helper statements first one pointing to your deployment server and the second one pointing to your DCP server.

USING DHCP OPTIONS

otherwise(Switch issues DHCP  but deployment server in the other vlan ) you have to use DHCP options to be able for your clients to discover deployment server.

Something like this in your switch DCP pool on the switches

ip dhcp pool VLAN100
network 10.5.206.0 255.255.255.0
default-router 10.5.206.1
option 66 ip 10.5.207.240
option 67 ascii smsboot\x64\wdsnbp.com

Or

ip dhcp pool VLAN100
network 10.5.206.0 255.255.255.0
default-router 10.5.206.1
option 66 ip 10.5.207.240
option 67 ascii smsboot\x86\wdsnbp.com

It is recommended to have a separate vlan segment for pxe boot as it would affect performance of your network and should think all the aspect,specifically broadcast and redundancy of pxe serves if you are deploying in a large environment

Regards,Prabath

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[jp.briggs]

Thank you for this descriptive answer. I apologize my description wasn't clear. The 2nd scenario is what we're looking at; i.e. The switch issues DHCP (IP, Def. GW, and name servers), and we want the deployment server to just issue the PXE parameters.

From your answer, it sounds as if Cisco does not support such a scenario. That is, you cannot define dhcp pools, and still use ip helper-addresses for Windows deployment.

Thanks again,

[Prabath Godevithanage]

So you have VLAN 100 which is the pxe boot or user vlan and then vlan105 which is the server(deployment) vlan and the dhcp server is another L3 switch.

In that case you can have two helper statements as below

interface Vlan100
ip helper-address X.X.X.X (deployment server)
ip helper-address X.X.X.X (DHCP Sever,L3 switch in your case)

here is an excellent article explaining the discovery process

We've been using PXE boot through DHCP option for a while now and never had issues with our setup,though You may have noticed that Microsft officially doesn't support DHCP options for PXE boot.

[jp.briggs]

Wait, so would this setup work ON the actual L3 switch acting as DHCP server?

I.e. On the L3 switch acting as DHCP server, there is a VLAN100, and on that VLAN100, I add an ip helper-address of the IP address of VLAN100 of the same switch - pointing to itself essentially?

I hope I'm explaining the question sufficiently. Our access layer is L3, so each access switch acts as DHCP server for all SVIs on that switch. So following your example above, on that L3 access switch, we would have:

interface Vlan100
 ip address Y.Y.Y.Y 255.255.255.0
 ip helper-address X.X.X.X (deployment server)
 ip helper-address Y.Y.Y.Y (Vlan100 SVI on this very switch)

And then also on this same L3 access switch, a DHCP pool defined for Vlan100:

ip dhcp pool Vlan100_dhcp_pool
 network Y.Y.Y.Y 255.255.255.0
 default-router Y.Y.Y.1
 dns-server <our dns servers>

I'll try adding the 2nd ip helper-address line on our SVI, and see if that has an effect.

Thanks,

[Prabath Godevithanage]

Hi,

"I.e. On the L3 switch acting as DHCP server, there is a VLAN100, and on that VLAN100, I add an ip helper-address of the IP address of VLAN100 of the same switch - pointing to itself essentially?"

You don't need the ip helper if the DHCP scopes are in the same switch as the SVIs.Switch will drop these packets anyway with the warning "Looping packet detected and dropped".You've noticed in my above post I have assumed that your DHCP scopes are on another device

In your case,you'd need option 66,67 to adjust the DHCP discovery process(I'll leave you to read on the topic) and also ip-helper on the PXE client vlan to enable
so the selected udp services get forwarded as unicast which also enables the PXE client to find the specified WDS/MDT TFTP server and Image file in IOS DHCP Scope options as option 66 and 67.

I've attached a working config from my lab,see if it helps in your case

Good Luck

[jp.briggs]

It would appear Cisco does not support an environment consisting of IOS DHCP pools AND ip helpers for PXE booting, without resorting to DHCP options.

I found this bug from 2009: https://bst.cloudapps.cisco.com/bugsearch/bug/CSCsj56741, and it would appear that Cisco has no intention of addressing it.

It's a shame, considering Microsoft identifies the ip helper route as the preferred method. The only way we could accomplish this now, is to move all DHCP services off our IOS devices and onto a server.

[Prabath Godevithanage]

I am not sure whether it's really a bug or due to the way the DHCP works within IOS,I haven't looked enough to comment on that but you certainly can get your setup going with minor configurations to your devices(please see my previous reply)

You might want to weight out which way is better for your network,If this is for to image up a device or two every now and then I would still stick with DHCP options without doing the major changes.

We've been running our non-DHCP integrated PXE environment with scope options setup for a while with no issues .mind you ,scopes are defined on the servers not on IOS same as you though