request for advice / building dhcpd infraestructure

[Leandro]

Simon , Bob , Peter:
Thank for sharing your wisdom on this, there is something I still can 
not understand.
Maybe a picture can help, please take a look bellow.
On scenario 1  following should happend:
Relay1 will set Gi-addr=1.1.0.1 so dhcpd will pick a lease from pool 1 
and network 1.1.0.0/24 will use 1.1.1.1 as gateway.
Relay2 will set Gi-addr=1.1.2.1 so dhcpd will pick a lease from pool 3 
and network 1.1.2.0/24 will use 1.1.2.0 as gateway.
This is ok;

On Scenario 2
After running out of /24 ips I will add remaining /24 networks behind 
each relay.
Add pool2 behind relay 1
Add pool4 behind relay 2.
How will it work ?
My concern is that, if relay1 still using 1.1.0.1 for gi-addr ; dhcpd 
can pick a lease from pool4 instead using pool2 since 1.1.0.1 falls into 
the 1.1.0.0/22 declared on shared network.
If this happens the request will receive the option router = 1.1.3.1 
witch ip is not set at any interface on relay1.
And vice-versa, dhcp server can pick a lease from pool2 for a request 
coming  from relay2, so it will also receive an incorrect router value.

relay


bw: Thanks Simon for  reading material sugestion, i will begin now.
Thanks in advance.
Leo.




On 23/06/15 18:19, Simon Hobson wrote:
> Leandro <ingrogger at gmail.com> wrote:
>
>> After I run out of those ips, I can do two thinks:
>> a)change the network mask from /24 to /23.
>> b)Add a second /24 subnet behind the relay , for example 1.1.2.0/24 and set a second gateway ip 1.1.2.1/24.
>>
>> option a) is not good since the broadcast domain at /23 could bring many collisions. (its just my opinion).
> I don't think it will make much difference. Don't forget that having two subnets in a shared network won't segregate the broadcast traffic. I think the only reduction would be from inter subnet traffic going via the router rather than using ARP to find the neighbour - but routing the traffic via the router rather than directly will more than outweigh any saving there.
>
>> option b) Could work but, how does relay agent knows witch ip to use for GI-Adrr ?
>> Can relay agent send both or more than one ips, on the GI-Addr field so dhcpd can figure out from witch range can serve the ip ?
> As already mentioned, as long as the GI-Addr value is within any of the subnets, then the server will work it out from the shared-network.
>
> BTW - I'd suggest a read of "The DHCP Handbook" by Ralph Droms and Ted Lemon, it explains all this and a lot more, and is quite readable.
>
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