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PXE Boot doesnot work unless helper-address is the first on the intrf

rick505d3
Level 1
Level 1

Hi,

I have a Cisco 2951 router (running 15.2(4)M3) which is also acting as DHCP server for the LAN. The L3 interface config is as below (the dhcp pool and excluded address config is configured correctly and remains the same for all cases)

interface BVI38

ip address 10.5.175.2 255.255.255.0

ip helper-address 10.4.1.158

ip helper-address 10.1.5.101

Unless the PXE Server address (10.4.1.158) is the first as the helper-address, the client computer doesn't get the TFTP download from the server. I tried different combinations even fake helper-addresses that doesn't exist on the network (1.1.1.1) along with the PXE server address - still unless PXE server is the first in the list, the client is unable to download the tftp file.

Does the position of PXE server address in the list of helper-addresses really matter ?

Regards,

Rick.

1 Reply 1

Peter Paluch
Cisco Employee
Cisco Employee

Hi Rick,

The ordering of the ip helper-address command should not matter if the DHCP servers are configured either identically (if both serve the same (V)LANs) or if each of them serves only a non-overlapping set of (V)LANs and does not send DHCPNAKs if received a DISCOVER/REQUEST message from a (V)LAN it does not serve.

I am suspecting that some timing condition is at the core of your observation. It would appear that the first IP address in the list of ip helper-address commands is the first to receive the forwarded broadcast. If it responds soon enough, it will beat the response of the second server that receives the forwarded broadcast just a few moments later. Clients usually choose the first acceptable response to their DHCP query, so if the first reply happens to miss the required PXE attributes or contains incorrect information necessary for PXE, the process of PXE boot will fail.

I would therefore suggest double-checking the configuration of both your DHCP servers. In particular, I would make sure to actually capture the DHCP OFFER messages sent by both servers and compare their contents to see if they contain identical information related to PXE booting. I would also keep an eye on negative responses from servers that could throw clients off track.

Best regards,

Peter

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