Static and dynamic DHCP across shared networks
Turnpaugh , George
George.Turnpaugh at arrisi.com
Tue Oct 15 20:51:41 UTC 2013
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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