Normally we have following scenario: The client PCs are in on subnet, a.e. 18.104.22.168 and the DHCP server in anoter subnet, maybe 22.214.171.124. So we configure the ip helper-address on the ethernet interface of the 126.96.36.199 subnet so that all DHCP broadcast traffic will be forwarded to the DHCP server which is located in tje 188.8.131.52 subnet. Now we have following situation and problems occur: The clients are in one subnet e.g. 184.108.40.206 and the server in another subnet, e.g. 220.127.116.11. But in this environment, we use 2 routers. The first (MSFC) one has configured the DHCP relay agent, but no interface in the subnet where the DHCP server is located. So this router has to sent the broadcast packets from the clients as unicast packets to a second router which has interfaces in both subnets. In this environment we have the problem, that the clients doesn''t get an ip address???
Try these things. Does the router in the 18.104.22.168 subnet have a route to the 22.214.171.124 subnet? Does the DHCP server have a default gateway set? Is there a scope on the DHCP server for the 126.96.36.199 subnet? Is the scope active? Does the router in the 188.8.131.52 subnet have a route to the 184.108.40.206 subnet? If you statically assign an IP address to a workstation in the 220.127.116.11 subnet, can it ping the DHCP server? In other words; make sure that TCP/IP and routing are working correctly and that the scope has been created and is active.
I already checked the TCP/IP connectivity - no problem. If I configure the ip helper-address on the router which has an interface in both subnets - I see no problem.
btw. The MSFC, this is the router which has only 1 interface in the 18.104.22.168 subnet is redundant and is configured with HSRP with the second one. A few minutes ago, we changed the active router to the second one and vice versa and now the problems are gone. The MSFCs are running 12.1(8a)E3. So you know any software related bug regarding such a problem?
A few things come to mind. Did you assign an IP address to the HSRP? Did you configure the IP helper address on both routers in your HSRP pair? Do you have an ARP cache issue? Is your physical configuration USER <---> HSRP routers <---> router2 <---> DHCP server? Is HSRP functional (show standby)? If I understand your physical topology correctly, two things are puzzling. 1) The helper address on router2 should not make a difference because the DHCP requests should be uni-cast packets at that point and should be routed to the proper subnet. 2) If HSRP is functioning properly, the active router should not make a difference. Can you post the configuration of the two HSRP enabled routers?
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