PXE with 3850

[slyfox]

Dear Community,

Please help to figure out how to properly configure PXE with 3850

DHCP role belongs to 3850 core ip 10.94.15.1
SCCM with PXE ip 10.94.10.6
Client network Vlan15

I tried this configuration, but clients stopped receiving ip addresses:

interface Vlan15
description "Office network"
ip address 10.94.15.1 255.255.255.0
ip helper-address 10.94.15.1
ip helper-address 10.94.10.6
ip pim sparse-dense-mode

interface Vlan10
description "Server vlan"
ip address 10.94.10.1 255.255.255.0
ip pim sparse-dense-mode

[Mark Malone]

Hi
All you need for PXE on a switch is the helper address pointing to the DHCP server ,and the switch needs to be able to reach the DHCP server , you have the correct config on the switch for PXE thats all thats required so something else is going on here , maybe on server side ?

[slyfox]

@Mark Malone 
The role of DHCP server is performed directly by the switch itself.
I must specify the ip helper of DHCP of the interface for this vlan?

[Mark Malone]

helper is only required if the DHCP server is on another subnet and it changes packet from broadcast to unicast and sends it on , if the switch is doing the DHCP and server is located on same switch its not required , the next step would be to debug the DHCP detail and see if the requests offers are been seen by client and server , one of them maybe dropping the DHCP packets

This may help with TSHOOT

Understanding and Troubleshooting DHCP in Catalyst Switch or Enterprise Networks

https://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/27470-100.html

[Richard Burts]

We do not have enough information to be able to really solve this question. I do have several comments and suggestions that I hope will be helpful:

- since the 3850 is doing the DHCP then you certainly do not need a helper address under the vlan interface pointing to itself

interface Vlan15
ip address 10.94.15.1 255.255.255.0
ip helper-address 10.94.15.1

So remove that helper address command.

- since SCCM is in a different vlan you may need the helper address for it

ip helper-address 10.94.10.6

So leave this command in the config.

- If we do not believe that there are problems with the configuration of the helper commands then perhaps the problem is with the parameters in the DHCP scope to support PXE. Can we see the configuration of the DHCP scope?

- I am not sure what this statement is telling us

I tried this configuration, but clients stopped receiving ip addresses:

Is it that the client was able to boot but did not receive an address? Or is it that the client was not able to complete the boot process?

HTH

Rick

[slyfox]

@Richard Burts 

ip dhcp pool Office network

network 10.94.15.0 255.255.255.0

default-router 10.94.15.1

dns-server 10.94.2.1 10.94.2.2

lease infinite

the client was not able to complete the boot process

If we leave only ip helper-address 10.94.10.6, does this mean that all requests for DHCP will only come to 10.94.10.6?

[Richard Burts]

Thanks for the additional information. There are several things to address.

- If the clients are in vlan 15 they will send their DHCP requests as broadcasts and the switch vlan interface 15 at 10.94.15.1 will receive the DHCP request and used the configured scope to assign IP addresses to the client. There is no need for a helper address pointing to 10.94.15.1.

- The helper address for 10.94.10.6 is so that clients can access the SCCM server which is need for the PXE processing. When your boot process needs to access a resource in a remote subnet you need the helper address. So you do need the helper address for 10.94.10.6 but not for 10.94.15.1.

- I see in your scope that you assign the lease as infinite. That makes me uncomfortable because once assigned an IP assignment never expires. That could lead to a situation where all of the available addresses have been assigned. (with 254 addresses in the pool it might take quite a while. But the possibility of the issue would be a concern to me. You can make the lease very long, but it is better if the lease does expire at some point)

- The configured DHCP scope is a pretty normal scope. It has statements for the network, for the default router, and for dns server. But I do not see any statements with the options needed for PXE boot. If there are no parameters for PXE boot then the clients do not know that they need to access the SCCM server and PXE boot will not work.

HTH

Rick

[slyfox]

@Richard Burts 

recommended config from Microsoft is:  When a client comes with a PXE Request, it receives first an IP Address from the DHCP, the DHCP request is also received by the DP and it provides the correct bootimage according to the header of the request (BIOS or UEFI). In this constellation, we don’t need the DHCP Options.

Is this really true or options 66 and 67 still required?

[Richard Burts]

I have worked with PXE a few times but do not claim to be expert about it. Perhaps someone else in the community who does have this expertise might jump in with an explanation. I can only say that I have seen PXE work successfully when options 66 and 67 are configured and I have never seen it work without them. My suggestion is to configure them. Or you can wait and see if someone else will provide a different explanation.

HTH

Rick