[c-nsp] DHCP Forwarding Strategy

[Bunny Singh]

I totally agree with Josh, as for the vlan 10 the gateway is 10.10.10.1 and with the help of configured ip helper-address, vlan 10 will send a unicast packet to the DHCP server.

Upon getting request on the DHCP server, it will search for free IP on the same IP POOL 10.10.10.1 and allocate the same to the user via vlan 10.

Regards
DS 


________________________________
 From: Joshua Morgan <joshua.morgan at gmail.com>
To: Markus H <hauschild.markus at gmail.com> 
Cc: "cisco-nsp at puck.nether.net" <cisco-nsp at puck.nether.net> 
Sent: Sunday, March 10, 2013 4:51 AM
Subject: Re: [c-nsp] DHCP Forwarding Strategy
 
What you're trying to achieve (mapping a client's request to a source VLAN
so that the DHCP server knows which pool/scope to serve from) is done out
of the box - the DHCP relay (configured using the ip helper-address
interface configuration command) will insert its IP address of the
interface that it received the request on into the DHCP giaddr field. It's
this giaddr field that is used to select the appropriate pool/scope.

So, as an example - you have Vlan10 SVI configured on your 3550 with ip
helper-address configured. Vlan10's IP address is 10.10.10.1. When a DHCP
packet is received by that interface, the switch will insert 10.10.10.1
into the giaddr field and relay it via unicast to the configured ip
helper-address. The DHCP server will read the giaddr field and pick the
appropriate pool/scope (so it would select a 10.10.10.0/24 pool/scope, for
example, as that matches the giaddr field).

Hope that helps,

Josh


On Sun, Mar 10, 2013 at 12:07 AM, Markus H <hauschild.markus at gmail.com>wrote:

> I've just read the Option 82 topic which brings to memory an unsolved
> prolbem I had:
>
> The "current" state of a network for a rather small event:
> At the core a Cisco 3550, a few 2950s and non-Cisco WLAN APs (the APs
> just map a few SSIDs onto the corresponding VLANs with either no
> encryption (guest) or WPA2 (event organziers)).
> The DHCP Server (dnsmasq) is a VM which has an interface for every
> VLAN where DHCP leases are needed (that would be all except the
> management VLAN für the ciscos and APs).
>
> What I would like it to be:
> The 3550 forwards DHCP requests to the DHCP VM. The VM decides (based
> on which VLAN the client is in) from which pool to give out an address
> and needs only one interface/ip. Further benefit: the VM server needs
> to deal with fewer VMs and gets a much more simple config.
>
> The Problem:
> As I understand Option 82 won't help, because port numbers on the
> cisco are not of much use in my case. And I haven't found a way to map
> the forwarded request onto a VLAN or subnet.
>
>
> So is there anything I am missing? Is there any good documentation on
> what information forwarded DHCP requests have by default or what
> things I can add (besides the quite useless port number where the
> request came in)?
>
> Greetings,
> Markus
>
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