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PXE Boot

Posted on 2014-01-13
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Last Modified: 2014-02-13
Setting up a PXE boot for our SCCM server.  Reading Microsoft best practices, they indicate not to use the DHCP options and setup a IP-Helper address on the internal router.  We currently have Cisco equipment in our infrastructure but I am not a network engineer.   How do I do this?
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Question by:WPincusIT
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vivigatt earned 500 total points
ID: 39782215
You need an iphelper (dhcp relay) only if your DHCP and PXE server(s) are not on the same subnet as the DHCP/PXE clients.
Is it your case?

Some resources:
My article (and the comments I made to it) dealing with DHCP options and PXE clients:
http://www.experts-exchange.com/Networking/Misc/A_2978-PXEClient-what-is-it-for-Can-I-use-PXE-without-it.html

http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol#DHCP_relaying

http://www.cisco.com/en/US/docs/ios-xml/ios/ipaddr_dhcp/configuration/12-4t/config-dhcp-relay-agent.html
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by:piwowarc
ID: 39801555
Hello

In our organization we tried achieving similar thing using ISC based DHCP and option on its own did not work. We had to put ip-helper pointing to SCCM server to make it working. Hope that is helpful.
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by:vivigatt
ID: 39801595
ip-helper is needed only to forward DHCP DISCOVER message to a DHCP or PXE server that is not on the same subnet than the PXE client.
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by:piwowarc
ID: 39801609
Hello

I actually know that. We manage over 80 DHCP servers for more than 300k users.

I am just saying when this thing started working. I may have been unclear. This ip helper is additional one to existing pointing to DHCP servers. MS has same advice in their support doc.

Not saying it makes sense, just the way it is.
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by:vivigatt
ID: 39801651
It does make sense actually, since a PXE server monitors DHCPDISCOVER messages and had to receive them.
If the PXE server and DHCP server are running on the same server, PXE service will be bound (listening) on port UDP 4011. PXE client must then know to use that port on this server and this is set by DHCP option 60, that must then be set to send "PXEClient". In that case, if the PXE/DHCP server is on another subnet than the PXE client, an ip-helper address must be set for DHCP.
Now if the PXE servcie runs on another machine than the DHCP service, the DHCP DISCOVER packets that the PXE Client sends to broadcast must also be received by the PXE service. Thus you also need an ip-helper address for the PXE (SCCM) server .

Check the following article that describes a PXE handshake between PXE client and SCCM PXE Service:  
http://blogs.technet.com/b/dominikheinz/archive/2011/03/18/sccm-pxe-network-boot-process.aspx

This other article may also help understanding what happens during PXE-boot:
http://blogs.technet.com/b/dominikheinz/archive/2011/03/18/dhcp-amp-pxe-basics.aspx
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