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[EX] Connecting PXE Hosts to Microsoft RIS (Remote Installation Service) Server via EX switch which is acting as a DHCP Relay agent

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Article ID: KB20630 KB Last Updated: 05 Mar 2017Version: 3.0
Summary:
This article provides information on how to configure the EX switch as a relay agent in an environment that has DHCP and Microsoft RIS on separate systems.
Symptoms:
The process of initial communication between PXE clients and RIS servers can differ, depending on how RIS is deployed with respect to DHCP services.

Microsoft RIS:

RIS uses the new Preboot Execution Environment (PXE) extensions to DHCP to initiate the installation of an operating system from a remote source to a client's local hard disk. The PXE environment is built on a foundation of Internet protocols and services that are widely used in the computer industry. These include TCP/IP, DHCP, and TFTP. The PXE extensions to DHCP protocol allows for information to be sent to network-bootable systems and it allow these systems to find RIS servers.

RIS uses DHCP as part of what is defined in the PXE architecture to initiate the process of remotely installing an operating system on a client. In other words, as PXE uses DHCP, so does RIS.

When a new DHCP PXE-based remote boot client is turned on for the first time, the client requests an IP address and the IP address of an active RIS server via the DHCP protocol and the PXE extensions to the DHCP protocol. As part of the initial request, as a DHCP option, the client sends out its GUID, which is used to identify the client in Active Directory. The client receives an IP address from the DHCP server and the IP address of the RIS server that services the client. In the RIS server's response, the client is given the name of a boot image that it must request when contacting the RIS server for initial service.

When the client makes its initial request for service, TFTP is used to download the boot image file to the client. In the case of RIS, this file is Startrom.com. Startrom.com prompts user to press the F12 key. If the user presses the F12 key, Startrom.com uses TFTP to download OSChooser and presents the user with the Client Installation Wizard.
Cause:
 
Solution:
If you have DHCP and RIS on separate systems, the initial interaction between PXE clients and RIS/DHCP servers will be as follows:

  • DHCP discover from client (asks for IP address and PXE boot server).

  • DHCP offer from DHCP server (offers IP address and other network configuration settings).

  • DHCP offer from RIS server (offers PXE boot server).

  • DHCP request from client to DHCP server (requests IP address).

  • DHCP acknowledge message from DHCP server (you can have this IP address).

  • DHCP request from client to RIS server (requests the boot server).

  • DHCP acknowledges the message from the RIS server (this acknowledgment contains the address to the RIS server and the first file that the client needs to send a TFTP request to start the boot process).

Topology
:

[ PXE Boot Clients ]  ------------- [ EX Switch] -------------- DHCP Server
                                      |                      
                                      |                                        
                                      |
                                  RIS  Server
  • PXE clients are connected to vlan4, vlan5, and vlan 6.

  • vlan.4, vlan.5, and vlan.6 are gateways for VLAN 4,5, and 6 respectively.

  • DHCP and RIS server is connected on VLAN 50.

  • The DHCP IP address is 10.0.50.6 and the RIS server IP address is 10.0.50.9.

The following configuration is required on the EX switch:
edit forwarding-options helpers
bootp {
     interface {
         vlan.4 {
             server 10.0.50.6;
             server 10.0.50.9;
          }
         vlan.5 {
             server 10.0.50.6;
             server 10.0.50.9;
         }
         vlan.6 {
             server 10.0.50.6;
             server 10.0.50.9;
         }
    }
}
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