Static and dynamic DHCP across shared networks

[Matt Bernardi]

If I'm following correctly it looks like this should fix your problem. CMs in there DHCP REQUEST will send their vendor class identifier as docsis, then the version. So a DOCSIS3.0 modem would be docsis30. So we use classes, see below. MTAs do the same but use pktc, then version. Once you have the below in dhcpd.conf just allow/deny classes per pool.  


# all docsis compliant cable modems begin their vendor class identifier
# with "docsis" this allows us to identify them as cable modems and 
# assign them to the right class
class "docsis" {
        match if substring (option vendor-class-identifier, 0, 6) = "docsis" and substring (option vendor-class-identifier, 0, 6) = "docsis";
}


# all packetcable MTAs identify themselves in the same way as 
# cable modems.  They begin their vendor class identifier with 
# "pktc"
class "pktc" {
        match if substring (option vendor-class-identifier, 0, 4) = "pktc";
}



# address pool for cable modems
	pool {
		allow members of "docsis";
		range 10.x.x.x 10.x.x.x;
              }

# address pool for MTAs
	pool {
		allow members of "pktc";
		range 10.y.y.y 10.y.y.y;
              }

# address pool for CPEs
# this includes anything that is not a cable modem or mta
           pool {
                deny members of "docsis";
                deny members of "pktc";
		range 10.z.z.z 10.z.z.z;
		}




On Oct 15, 2013, at 4:51 PM, "Turnpaugh , George" <George.Turnpaugh at arrisi.com> wrote:

>  
> 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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