dhcp-users Digest, Vol 31, Issue 15

Glenn Satchell glenn.satchell at uniq.com.au
Sun May 8 03:05:38 UTC 2011

Yes, you have it correctly.

There is a third option (and the more general case): the relay agent box 
has two interfaces, one connected to the subnet of the dhcp client, and 
the other connected to a different subnet that still needs to route to 
the dhcp server.

In all cases the dhcp relay box needs to be able to route to the dhcp 
server, whether this is through either network interface, or because the 
relay box has an interface on the same subnet as the dhcp server.

When the two are on the same subnet there is an implicit route for the 
local subnet, so you don't need to add a specific route.

A simple test is "can the relay box ping the dhcp server?"


On 05/08/11 12:51, Derek Wang wrote:
> Hi Glenn,
> thanks for your quick response!
> I want to confirm with you about my understandings:
> 1). if the relay agent box has two interface, one is connected in the
> subnet of the dhcp client
> the other one is connected in the subnet of the dhcp server, then we do
> not need to explicitly
> specify the route, because the dhcp client use broadcast to send out the
> dhcp request, after
> the relay agent catch it, and then the relay agent generates new package
> to the dhcp server
> via unicast, as the relay agent has one interface connected in the dhcp
> server's subnet, so this
> works.
> 2). for the relay agent has only one interface, then the relay agent
> need to config the route explicitly,
> because the relay agent and the dhcp server are not in the same subnet,
> so it needs a way to route
> the new generated package to the dhcp server.
> the above are my understandings, if there are some misunderstanding in
> that, please correct!
> thanks in advance!
>     Message: 3
>     Date: Sat, 07 May 2011 21:28:03 +1000
>     From: Glenn Satchell <glenn.satchell at uniq.com.au
>     <mailto:glenn.satchell at uniq.com.au>>
>     Subject: Re: the question about the dhcp relay deployment
>     To: Users of ISC DHCP <dhcp-users at lists.isc.org
>     <mailto:dhcp-users at lists.isc.org>>
>     Message-ID: <4DC52CC3.2040804 at uniq.com.au
>     <mailto:4DC52CC3.2040804 at uniq.com.au>>
>     Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>     Usually not. The packets aren't routed directly through the box, they go
>     into the box to the relay process. The relay process then generates new
>     packets to the server.
>     Of course you can run the dhcp relay on a box with a single interface
>     too. It doesn't have to be the router for the network, it can be any
>     box, as long as it has a route to the dhcp server.
>     regards,
>     -glenn
>     On 05/07/11 11:37, Derek Wang wrote:
>      > Hi all,
>      >
>      > When the dhcp client and dhcp server are not in the same subnet,
>     we need to
>      > make use of the relay agent to bridge the dhcp client and dhcp
>     server.
>      >
>      > I have one question for that, do we need to enable the forwarding
>      > service on the
>      > relay agent machine? Or it's enough to just enable the relay
>     agent service?
>      >
>      > thanks!
>      > --
>      > Best Regards,
>      > Derek Wang
>      >
> --
> Best Regards,
> Derek Wang

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