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DHCP, Subnets

I have a network divided into subnets with each subnet being serviced by a variety of Cisco 2620, 2501, & 805 routers. In setting up a test of Win2K servers we placed 1 Win2K DHCP server in a subnet at a district office and another in a subnet at the main office. The district office server could serve IP addresses locally and the same was true for the server at the main office being able to serve locally. But the main office server could not service an IP request over the network thru the routers to the district office or vice versa.

We called our Cisco router tech and he said that this was normal that the routers were not allowing the client IP request broadcasts to pass thru. So he came out and reprogrammed the routers to pass the request broadcast thru. He did not direct the broadcast directly at our DHCP server, but only allowed the broadcast to pass. Well, this is not working!!!

While he was on site, he did test the case where he directed the IP request directly at our DHCP server and the server responded perfectly.

We would like to not program the routers to point directly at a DHCP server's IP but allow the broadcast to pass thru to the root subnet. This would allow us to have muliple DHCP servers at the root for redundacy with the district office server.

Is it possible on the Cisco routers to pass the IP broadcast requests straight thru the routers to the root subnet? Or will we have to have the routers programmed to direct the IP broadcast requests straight to a single DHCP server IP?

Thanks for all of your help,


  • Other Network Infrastructure Subjects

Re: DHCP, Subnets

Hi, your Cisco router tech probably have configured

"ip helper-address ".

You can have multiple entries of this command. Like:

"ip helper-address "

"ip helper-address "

However, "ip forward-protocol udp " should be used

to ensure that only the desired UDP port will be forwarded.

"ip forward-protocol udp 67"

"ip forward-protocol udp 68"

(ports 67 and 68 are used for BOOTP)

(DHCP is carried inside BOOTP packets)

Another option you can try is defining a broadcast address

instead of specific DHCP address. (For the reason that you

might be changing the DHCP address without the expense of

changing the router configuration)

Try using the following command under the interface config mode:

"ip helper-address "

"ip directed-broadcast"

(e.g., using /24 "ip helper-address")

But take note that enabling directed broadcasts could be a

security hole.

Hope this helps.

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