Static and dynamic DHCP across shared networks

[Neff, Glen]

I didn't go over your config with a fine toothed comb, but one thing that's glaring, is that static assignments must be outside any range.  I know this seems a little counter-intuitive, and I struggled with it years ago, but that's how it's got to be.

-G

/*
 * Glen R. J. Neff
 * USD Lab Operations Infrastructure Team
 * glen.neff at emc.com
 *
 * EMC^2 == E^2
 */

________________________________
From: dhcp-users-bounces+glen.neff=emc.com at lists.isc.org [dhcp-users-bounces+glen.neff=emc.com at lists.isc.org] On Behalf Of Turnpaugh , George [George.Turnpaugh at arrisi.com]
Sent: Tuesday, October 15, 2013 4:51 PM
To: dhcp-users at lists.isc.org
Subject: Static and dynamic DHCP across shared networks


We're trying to create a dhcpd.conf file for multiple sub-networks such that we can control IP address assignment for cable modems and CPE’s connected to these cable modems.

Each sub-network contains several host entries, each with a fixed IP address. (These are for the cable modems.)
Each sub-network is configured as a /19 network.

All of the sub-networks are contained within a "shared-network" clause.
The first sub-network is considered the default network.

An example of our dhcpd.conf file is shown below.
We found that the "shared-network" statement is needed to get the specified fixed addresses to be handed out, otherwise only addresses from within the first sub-network are picked.


shared-network MVRF1 {

   # Subnet 1
   subnet 10.106.0.0 netmask 255.255.224.0 {
      range 10.106.0.255 10.106.31.254;
      option subnet-mask 255.255.224.0;
      option broadcast-address 10.106.31.255;
      option routers 10.106.0.1;
   }

   # Subnet 2
   subnet 10.106.32.0 netmask 255.255.224.0 {
      range 10.106.32.255 10.106.63.254;
      option subnet-mask 255.255.224.0;
      option broadcast-address 10.106.63.255;
      option routers 10.106.32.1;
   }

   # Subnet 3
   subnet 10.106.64.0 netmask 255.255.224.0 {
      range 10.106.64.255 10.106.95.254;
      option subnet-mask 255.255.224.0;
      option broadcast-address 10.106.95.255;
      option routers 10.106.64.1;
   }
}


#Vrf_Default
group {
   host modem10 {
      hardware ethernet 00:00:ca:3a:1b:63;
      fixed-address 10.106.1.20;
      filename "basic.bin";
   }
   host modem11 {
      hardware ethernet 00:00:ca:3a:1b:60;
      fixed-address 10.106.1.21;
      filename "basic.bin";
   }
}

#Vrf_A
group {
   host modem20 {
      hardware ethernet 00:00:ca:3a:1c:8c;
      fixed-address 10.106.33.20;
      filename "basic.bin";
   }
   host modem21 {
      hardware ethernet 00:00:ca:3a:1c:ce;
      fixed-address 10.106.33.21;
      filename "basic.bin";
   }
}

#Vrf_B
group {
   host modem30 {
      hardware ethernet 00:00:ca:3a:20:61;
      fixed-address 10.106.65.20;
      filename "basic.bin";
   }
   host modem31 {
      hardware ethernet 00:00:ca:3a:14:8b;
      fixed-address 10.106.65.21;
      filename "basic.bin";
   }
}


There are two problems we're trying to find a solution to:

1. When a cable modem WITHOUT a fixed address registers, the DHCP server dynamically hands out the highest free address from the "highest address shared sub-network".
In the above example, it's from sub-network 10.106.64.0, (for example 10.106.64.254).
The address handed out is NOT from the first sub-network defined (10.106.0.0), which is our default sub-network.


-          Is there a way to specify which sub-network addresses are picked from for dynamic hosts within a shared network space.


2. When CPE's connected to a cable modem request DHCP, they also end up getting an address in the same way described above,
they get the highest free address from the "highest address shared sub-network".
This occurs even though the relay agent address (for example 10.106.32.1, as verified by Wireshark) is an address from within the sub-network upon which the modem is registered.


-          Is there a way to have the DHCP server dynamically hand out a free address from within the sub-network upon which the modem is registered.

Thanks in advance for anyone who may have knowledge in this area,
G. Turnpaugh

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