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Routing PXE TFTP packets

Posted on 2010-08-19
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Last Modified: 2013-12-12
Hi,

I'm trying to get a Linux workstation to install using autoyast (PXE and TFTP) across two VLANs.

My DHCP and PXE server are on Vlan 100, and the server has the address 10.199.100.50
My PXE client is on Vlan103.

The client obtains a DHCP address correctly, but then the PXE agent times out with an "PXE-E11: ARP timeout" message.

How do i configure my router to route the TFTP packets?


Currently, it is configured thus:

interface Vlan100
 ip address 10.199.100.1 255.255.255.0
!
interface Vlan103
 ip address 10.199.103.1 255.255.255.0
 ip helper-address 10.199.100.50


Thanks,

k.
pxe.png
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Question by:Kevin_Leadbeater
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Expert Comment

by:Kvistofta
ID: 33477595
This does not sound like a routing-issue. "ARP timeout" means that the client send an arp request as a layer2 broadcast in order to get the MAC-address for someone, probably the gateway 10.199.103.1, but doesnt get a reply.

You need to troubleshoot physical connectivity and layer2-issues between this client and the 10.199.103.1-host rather than routing...

/Kvistofta
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Author Comment

by:Kevin_Leadbeater
ID: 33624727
There isn't a physical connection problem as far as i can see.  The IP helper address is correctly forwarding the DHCP broadcast queries (DHCPDISCOVER) to the server and the server in turn is responding with a DHCPOFFER message.  Then the DHCPREQUEST and DHCPACK message are sent and received properly.  Indeed as you can see in the screen shot, the client (on VLAN103) is receiving the correct IP address from the DHCP server.

The problem is that once it has the correct IP address, it needs to to a TFTP transfer (which seems to be done using broadcast, hence the ARP message).  This "ARP request" isn't being forwarded via the IP Helper Address.  And so the client isn't able to download the relevant installation files.

If i understand the TFTP protocol correctly, it is using UDP port 69, and the Cisco ip-helper address should be forwarding 8 UDP ports - 37, 49, 53, 67, 68, 69, 137 and 138.  

So my question is why can't the client download the tftp boot files?
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Kevin_Leadbeater earned 0 total points
ID: 33902688
My problem was caused by a stupid error in the DHCP server where the default gateway was incorrect.
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