Instead of deploying a DHCP server on every subnet, discover how to install
and configure a DHCP relay agent. By far the hardest part of mastering the DHCP relay agent is installing it. Once
you have found the relay agent, configuring it to listen for DHCPDiscover packets, is the proverbial piece of cake.
DHCPDiscover packets, like all broadcasts, cannot pass across routers.
In fact that was a lie, if you have a modern Router which is RFC 1542 compliant,
then you can forward the DHCPDiscover packets to a DHCP server in a different
subnet. In this instance, the Router acts as a Relay Agent.
It is rare for Microsoft to remove functionality, but while NT 4.0
Workstations could act as DHCP Relay agents, XP and W2K Pro cannot. So you
need to install the relay agent on a Windows Server 2003.
What is not
obvious is where you find the relay agent, the answer is in Routing and Remote
Access. When you think about it, the relay agent is a type of router,
hence the RRAS location to install and configure the DHCP Relay agent makes
sense.
As
I say once you find and install the Relay Agent, configuring is easy, all you
need to do is tell the router or DHCP relay agent the IP address of the real
DHCP servers. Just right-click the DHCP Relay Agent, and then select
properties from the shortcut menu.
Trap: you forget to add an interface. See that 'ISP' interface in the screen
shot is Enabled.
Let us turn this tutorial to see how the Relay Agent works. What happens is the Relay agent intercepts DHCPDiscover packets from clients
and then unicasts to the DHCP on their behalf. The secret of successful
relaying is to create the appropriate scope on the DHCP server. The
first time it worked I thought that it was a miracle that the client got the correct IP
address. On reflection, I realized that the Relay Agent adds the Source IP
address when it contacts DHCP. So now I understand how the server knows,
from its list of scopes, which subnet to offer an IP address.
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How many routers lie between your client and its DHCP server? Each
router would represent 1 hop, so calculate the maximum hop count that you need and
configure the Relay Agent accordingly. From the Routing and Remote Access
interface, navigate to the IP Routing, DHCP Relay Agent, right-click the
Interface, not the server, and check the Hop Count threshold.
Boot Threshold
The boot threshold setting is for the cautious (or paranoid). Such
people would have a DHCP server AND a Relay
Agent on the SAME subnet. In these circumstances, you should consider how long the Relay Agent should
wait for the main DHCP server to respond. To do this adjust the Boot
threshold.
Where you have relay agents, especially if you configure more than one, there is a
possibility of duplicate IP addresses. The conflict detection feature
means that the DHCP server checks by pinging the proposed address lease before
actually issuing it. Naturally, if the server receives a reply that IP
address is not offered.
Conflict Detection is a property of the DHCP server as a whole and not
of individual scopes. To set the threshold, right-click the server icon,
properties, then Advanced (Tab).
If all else fails, then clients give themselves an Automatic IP address in
the range 169.254.x.y where x and y are two random numbers between 1 and 254.
Whilst APIPA is a sign of failure, the fact that the client has a valid IP
address means that it can keep on polling to see if a DHCP server has come back
online. In NT 4.0 days, a client would end up with a 0.0.0.0 address if
there were no DHCP server, and then you needed to reboot to obtain a valid IP
address.
When you set up a relay agent there are a number of other factors to
consider, check your, Hop Count, Boot Threshold, and conflict Detection. My tutorial compares the difficulty of installation with ease of configuring.
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