DHCP via a MS Domain Controller or Cisco Router.

[stamac706]

Hello everyone,

I'm starting my career as a Cisco engineer and have some doubts about certain infrastructure topics.

This is the situation I'm facing in the company I'm working at currently:

The company is replacing old routers and switches, and revamping the whole network infrastructure (cabling, racks, etc.). They're gonna keep their servers. The thing is that they have several Domain Controllers with AD, DHCP and other services centralized in one site, from there all of these services are distributed throughout the entire organization that has branch offices as well in different regions and separated buildings.

How can I do in order that the other remote users can get DHCP and others services from the central office through their respective routers because routers don't allow broadcasting, and how should I planned a future VoIP implementation where the new IP phones must have their IP addresses from a DHCP server? I'm really confused.

They wish to keep running DHCP from the servers to serve workstations and other devices. There are other intrincacies here, but I'd like to start with this one. Any other piece of information, please let me know.

I've been checking a lot of documentation on implementation, network infrastructures, and none mention anything like that. I'd like some detailed guidance on this please. I'd appreciate it.

Thanks in advance...

[Reza Sharifi]

Hi David,

The DHCP can be installed on a server as part of the AD and can also be located at the central location.  As long as the DHCP server is routed and reachable by the remote hosts, than they can obtain an IP address from it.  So, although it nice to have a local DHCP at each location, but it is not necessary.

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfdhcp.html

HTH

[darren.g]

David Santamaria wrote:
How can I do in order that the other remote users can get DHCP and others services from the central office through their respective routers because routers don't allow broadcasting, and how should I planned a future VoIP implementation where the new IP phones must have their IP addresses from a DHCP server? I'm really confused.

What you wnt is perfectly workable, and I do exactly that across half a dozen different sites - I have one DHCP server in the AD forest (centralised managment), and it allocates IP addresses across approximately 11 different subnets in 6 or 7 sites.

You need a couple of things to make this work.

The first is, obviously, complete connectivity from your client network to the DHCP server. If you can't PING the DHCP server, then you're not going to be able to get an IP address out of it!

The second is to configure a different scope in your Microsoft DHCP server for each site - be sure you get the network and subnet details correct, because it is this which the DHCP server uses to determine which pool to allocate the IP address from. The scope must, of course, be active - or the DHCP server won't allocate from it.

And lastly, you need the "ip helper-address" keyword defined on your routing interfaces at the remote sites - or its equivalent if you're using high end (Nexus) switches. On NXOS (Nexus switches), there are a couple of extra steps, and the keyword is different (ip dhcp relay address instead of ip-helper address), but the concept is identical.

The format for the "ip helper-address" command is pretty simple. Here's an example from one of my SVI's

interface Vlan4

description Network for VMWare virtual hosts

ip address 10.100.15.254 255.255.254.0

ip helper-address 10.100.12.2

end

I've obfuscated the addressing a bit, but you get the idea.

You'll need to configure that on the ROUTING interface for every subnet you want to have DHCP working on. In this case, it's configured on an SVI (switched virtual interface) in one of my layer 3 switches - but if you use a router, the concept is the same.

Cheers