IP helper-address doesn't work properly

[DuJin0509]

Hello everyone,

we recived complaints from customers that their PCs can't obtain IP address from remote DHCP server (the DHCP server and clients are not in same subnet), we confugred ip helper-address to let the client reaches the remote DHCP server, we have captured ip accounting output from router, the odd thing is we observed the DHCP discovery packet to the remote DHCP server is with 0.0.0.0 as source address rather than the Gi0/0 IP address 10.157.43.130, according to my knowage this source IP address should be the LAN interface IP address after we configured ip helper-address. seems the ip helper-address doesn't work properly to replace the original source IP address 0.0.0.0.

Any explanation to that? I'm at a loss :(

interface GigabitEthernet0/0
 ip address 10.157.43.130 255.255.255.224
 ip helper-address 10.157.44.7
 ip accounting output-packets
 ip nat inside
 ip virtual-reassembly
 duplex auto
 speed auto
 standby 10 ip 10.157.43.129
 standby 10 preempt
 standby 10 track 1 decrement 10

R1#sho ip accounting

   Source           Destination              Packets               Bytes

0.0.0.0          10.157.44.7                     25                 8200   >>>>>>>>>>>>>>>>>>>>>>>>>>

[2edc3rfc]

Is your router R1 able to reach the DHCP server on 10.157.44.7?

Is it possible to upload the running configuration so I can have a look.

[DuJin0509]

Hello Reubeil,

There is no connectivity issue at site and the remote DHCP server is pingable.

seems ip helper-address doesn't work.

I have no chance to get the whole configuration from the router due to audit issue.

What I could confirm is there is no firewall or ALC installed between router and DHCP server...

Just simply configured ip helper-address to the LAN interface that is the default gateway for LAN hosts...

[2edc3rfc]

I can see that you have HSRP enabled on the interface. Is this the primary? Is the DHCP helper added on the other device as well?

Try changing the dhcp-helper rule to the following

interface GigabitEthernet0/0
 ip address 10.157.43.130 255.255.255.224
 ip helper-address 10.157.44.7 redundancy Failover
 standby 10 name Failover

Do the same on the other device.

[Georg Pauwen]

Hello,

in addition to the other posts, try to globally configure:

ip forward-protocol udp 67
ip forward-protocol udp 68

on all routers between your hosts and the DHCP server.

Another usual suspect could be that the switchports where the hosts are connected to are not configured with 'spanning-tree portfast', you might want to check that as well...

[paul driver]

Hello

Apply the relay closest to the hosts that is to the access switchs svi interface

Also do you have dhcp snooping enabled - if so my sure you have trusted interfaces on interfaces towards the dhcp server and any interconnect s between switches

lastly is dhcp option 82 enabled and if so does the dhcp server support option 82 

res

paul

[Richard Burts]

The original post seems to assume that the helper address is not working based on observation of an accounting record that shows a source address of 0.0.0.0. I question the validity of this assumption. Bear in mind that with helper address the router is not sending the source address as its IP but is sending the gateway address (a field in the DHCP request). So I believe that seeing a source address of 0.0.0.0 is appropriate.

If users are not receiving IP addresses then there is some problem but I do not believe that it is necessarily that helper address is not working. Can the original poster confirm that the DHCP server does have an appropriate scope configured for 10.157.43.128? Can the original poster check the logs from the DHCP server and see if there are any log messages about requests from this subnet?

HTH

Rick

[DuJin0509]

Hello dear all,

Issue has ben resolved by enable service DHCP on the router,  this service shoule be enabled by default not suer who disabled it for what purpose , after this that customer comfirmed they could obtain IP address as usual, looks the DHCP relay agent has to work with service DHCP.

Thanks a lot for your kind support.

[Richard Burts]

Thanks for the update. It is good to know that you got it resolved and that the issue was that service dhcp was disabled.

HTH

Rick