request for advice / building dhcpd infraestructure

Bob Harold rharolde at umich.edu
Wed Jun 24 17:01:57 UTC 2015


On Wed, Jun 24, 2015 at 12:38 PM, Leandro <ingrogger at gmail.com> wrote:

>  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.
>
>
You need separate shared networks:
shared-network sitea {
subnet 1.1.0.0 ...
...
subnet 1.1.1.0 ...
...
}

shared-network siteb {
subnet 1.1.2.0 ...
...
subnet 1.1.3.0 ...
...
}



>
> On 23/06/15 18:19, Simon Hobson wrote:
>
> Leandro <ingrogger at gmail.com> <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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