In-Depth

Visit to a Far-Flung Subnet

In which Mole addresses issuing IP addresses to clients all over the place.

Dear Mole,
I’ve learned that DHCP uses broadcast protocol and will not be relayed through a router. We have around 43 locations spread out in the U.S. and have Windows NT 4.0 and Bay Networks routers for routing between the locations. The issue is that we need our NT server at our corporate site to issue IP addresses to our clients at all locations with different subnets. Need your advice.
Thanks,
—Senthil
Network engineer

Senthil,
Thinking out loud, Mole can come up with a couple of ways for your clients to obtain an IP address. The first way is to get the IP address from your router. For the sake of discussion, Mole is going to assume that your Bay Networks router is one of the Nautica 4000 series. The Nautica 4000 can be configured to function as a local DHCP server for workstations connected directly to the local Ethernet segment. This is fine if you’ve got only one segment, but you need one central NT server to hand out IP addresses to clients all around the U.S. (Letting several servers or routers on your network think that they’re the only ones that matter and handing out IP addresses willy-nilly will soon cause problems with duplicate IP addresses on the network. And we know we can’t have that, right?)

Now, DHCP is an extension of the initial Bootstrap Protocol or BOOTP, which assigned IP addresses from a database of physical addresses matching IP addresses. What you need to do is configure the routers to be BootP (Bootstrap Protocol) Relay Agents, called BootP Relay Mode on Bay Network routers. DHCP gives a framework for passing configuration information to IP hosts connecting to a network, as defined under RFC1541. BootP Relay Mode passes IP address requests and delivers host-specific configuration parameters from a DHCP server to a DHCP client.

A general overview of Bay Network’s Nautica 4000 Router can be read from Bay Network’s Nautica 4000 Web page at www.nortelnetworks.com/products/02/datasheets/2913.html.

So, to summarize, if you want to implement DHCP, and clients are unable to obtain IP addresses, and you have a multiple subnet configuration, then the following information applies to you.

A DHCP server can provide IP addresses to client computers spanning multiple subnets if the router that separates them can act as an RFC 1542 (BOOTP) relay agent. To proceed:

  1. Configure a BOOTP/DHCP Relay Agent on the client segment. This can be the router itself or an NT computer running the DHCP relay service.
  2. Configure a DHCP scope to match the network address on the other side of the router where the clients are. Make sure the subnet mask is correct. Don’t configure a default gateway on the DHCP server’s NIC such that it’s the same address as that of the router supporting the subnet where the clients are. Don’t include that subnet’s scope in any superscope configured on the DHCP server’s LAN segment.
  3. Make sure there’s only one logical route between the server and the remote clients.

For more information, read this KB article: “How to Configure Microsoft DHCP Server for BOOTP Clients (Q174765).

And if that doesn’t satisfy your information hunger, Mole suggests querying the Knowledge Base using the query “bootp”. A good backgrounder is a TechNet article, “Managing TCP/IP Addresses on Your Network with DHCP,” located at www.microsoft.com/TechNet/maintain/mgtcpip.asp.

You’ve got it knocked, man.

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