IOS DHCP, PXE and IP helper address

[tech]

I have a local IOS DHCP server with PXE clients and a remote PXE server. The DHCP server responds with an IP address and a next server IP address of the remote PXE server in the DHCPOFFER. The client never sends a request to the PXE server to obtain the bootfile. I have tried configuring the IP helper address for the PXE server as well as using the next-server DHCP configuration without luck. Since the PXE server can not listen to the DHCP request (since they are on different subnets) then I need a way to force the client to query the PXE server for a bootfile. Has anyone successfully created a PXE and DHCP server setup when the machines are on different subnets?

Thanks,

RJ

[thisisshanky]

Usually Next-servers are TFTP servers, i dont know if your client uses TFTP for booting or some other protocol. Put a sniffer (ethereal) on the segment and look at what udp ports (traffic) passes through that segment. YOu can forward extra UDP ports on the router using ip forward-protocol udp under global config mode. Here is some more info about PXE.

http://www.answers.com/topic/preboot-execution-environment

[tech]

Thanks for the reply. Our PXE clients use TFTP to download a bootfile and it is able to start the download but gets an error before finishing it.

Thanks,

RJ

[thisisshanky]

Can you paste what error you get ?

[tech]

Error PXE-E76: Bad or missing multicast discovery address.

Description - Multicast discovery is enabled but the multicast discovery address tag is missing.

I have other PXE clients on the same LAN as the PXE server but in different (adjacent) VLANs. The Layer 3 Vlan interface has the ip helper-address of the PXE server and also the ip helper-address of a DHCP server on the same subnet as the PXE server. This setup works fine so I know it is not a problem with the bootfile. It is related to the setup of the PXE and DHCP servers on different subnets.

Thanks,

RJ

[thisisshanky]

I think your PXE clients are using MTFTP which uses Multicast. Again this traffic wont be forward using helper-address as its not a broadcast. ITs a multicast. YOu will need to enable multicast forwarding on your router to forward this multicast TFTP traffic to the PXE server which holds the bootfile. It works in the adjacent vlan, as the clients are on the same LAN as the PXE server. Can you put a sniffer such as ethereal and filter multicast packets and see what Multicast packet on port 69 is used (TFTP). THen you can enable PIM routing to forward multicast traffic from this vlan to the PXE server vlan.

Here is a guide to multicast routing.

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca794.html#wp1000948

Here is a link to ethereal - download.

http://www.ethereal.com/download.html