DHCP (NT) allocation address problem.

[serge.guilbert]

We have #2 ciscos 7200 with HSRP running, connected

to #2 network locations thru ATM link.

We are using NT DHCP centralized server in a remote location (2 hops).

The problem was that when we was requesting an address , we got a time out, and was obliged to

start a ipconfig/renew at least three times before

having a correct address delivers by the DHCP server.

Finally we solve the problem in putting the address scope subnet in primary address on the fastethernet interface of the router, before the address scope subnet was define on the router in the the secondary address.

But I would like to understood why it was not working

before and it works now, what really the difference,

is there something who was not well configured ?

was recognize in a su

[ccie5851]

When you use the ip helper-address commands on an interface with secondary addresses, the DHCPREQUEST packet that is forwarded to the DHCP server has a source network address of the primary address. The best way (I have found) to make life easier is not use secondary addresses. They are a real bear to troubleshoot and work with.

You can always use your router as a DHCP server, but I personally do not recommend/implement this.

[serge.guilbert]

the DHCP server is a microsoft DHCP server.

For me having the DHCP sunet scope in the primary or

the secondary address subnet should me the same thing.

[jwitherell]

If I understand what you're asking, I had a similar situation recently. In my case, I had a primary and secondary address on a subnet with a Cat5500 RSM between that VLAN and the VLAN the DHCP server was on. I reversed the primary and secondary addresses (for another reason) and ended up "breaking" DHCP for that subnet. After getting with our Unix guru and looking at his logs, we learned that when the router forwards the DHCP request to the helper address, the DHCP request packet has a "hint" with the router interface's primary ip address subnet as the source. So, if using secondary IP addresses on your router's interface, DHCP will only work for workstations on the same IP subnet as the router's *primary* IP address. Any secondary addresses won't have DHCP services.