Master server, multiple cisco relays, many subnets.

[Simon Hobson]

Matt Jenkins wrote:

>I manage the network for a WISP and need to migrate away from my 
>Cisco routers serving DHCP. I think I have an understanding of what 
>I need to do to configure a central DHCP server but I am still a 
>little lost after reading the archives, man pages, example conf 
>files, etc. I have setup dhcp servers before on most variations of 
>Linux/BSD/Windows. I am also very knowledgeable with Linux/BSD for 
>8+ years now. What I need help with is the giaddr identification 
>that each cisco router will pass to the Master server.

This **should** be automagic.


>For a small example (All Ips are imaginary):
>
>Master Server
>Eth0 12.1.1.10
>Eth0:1 10.10.0.88
>
>Cisco 1
>Serial0 88.15.15.89
>FastEthernet0
>- 21.17.21.1
>- 21.17.22.1
>DHCP Ranges
>21.17.21.0/24
>21.17.22.0/24
>
>
>Cisco 2
>Serial0 65.45.158.48
>FastEthernet0
>- 52.84.19.1
>- 68.54.78.1
>DHCP Ranges
>52.84.19.0/24
>68.54.78.0/24
>
>I want the DHCP server to handle all those dhcp ranges but only 
>relay them to the correct cisco router. Is this possible?

Yes it's possible - it's standard functionality.

Just to clarify, taking the first router :
>Cisco 1
>Serial0 88.15.15.89
>FastEthernet0
>- 21.17.21.1
>- 21.17.22.1
>DHCP Ranges
>21.17.21.0/24
>21.17.22.0/24
Are these two subnets on the same wire ? Ie
ip address 21.17.21.1 255.255.255.0
ip address 21.17.22.1 255.255.255.0 secondary

In this situation, this is what's called a shared network, and your 
config for this network would be :

shared-network "cisco-1" {
   subnet 21.17.21.0 netmask 255.255.255.0 {
     option routers 21.17.21.1 ;
     range 21.17.21.2 21.17.21.254 ;
   }
   subnet 21.17.22.0 netmask 255.255.255.0 {
     option routers 21.17.22.1 ;
     range 21.17.22.2 21.17.22.254 ;
   }
}

Be aware that by defaults, all addresses in both subnets are equal - 
a client could get an address from either. This is a fundamental 
limitation of the broadcasts used in DHCP.
I'm not certain, but I think the router will use the primary IP 
address as the GIAddr in forwarded packets. The DHCP server sees that 
this is in part of a shared network and will automatically hand out 
addresses in both subnets.


On the Cisco routers, it's a matter of turning on the IP Helper :
   ip helper 12.1.1.10
however, this also tuns on a load of other things that you almost 
certainly don't want (thanks Cisco !). I can't recall what they are, 
but it will be in the archives if you search for "ip helper".