Regarding option routers

Vithal Shirodkar vithals at
Fri Jul 14 01:41:52 UTC 2006

Thanks a lot Simon.  The following statement did the trick
option routers packet = (24,4) ;

Appreciate your help.
- Vithal

>From: Simon Hobson <dhcp1 at>
>Reply-To: dhcp-users at
>To: dhcp-users at
>Subject: Re: Regarding option routers
>Date: Thu, 13 Jul 2006 16:44:08 +0100
>Vithal Shirodkar wrote:
> >Thanks for the detailed response and the note of caution.  I am running 
> >experiments with provisioning of multiple networks on the same set of
> >hardware (routers, server, etc) and so client reconfiguration is not an
> >issue, since when router gets a new config, the servers reboot too.
> >
> >Can you explain why you said the ISC server won't work if there are no
> >shared subnets ?
>I didn't say that ! I said that it won't do what you want - ie take
>the relay agent address and use that as the router address in any
> >Can't I have the dhcp server say on subnet
> >connected to a router which also is connected to 2 other subnets 
> >and and a helper configured in each of those subnets pointing 
> >the dhcp server on
>Yes you can.
> >Or have I just described a shared subnet ?
>It depends - your description isn't clear enough to say ! I'm not
>clear whether you are talking networks or subnets above - you only
>need one relay agent per network because when a client does not yet
>have an address to use, it broadcasts to the all ones broadcast
>address which doesn't care about what subnets are defined.
>In a basic network without shared subnets, it would be possible to
>use the relay agent address as a gateway address no server I am aware
>of will do it automatically for you.
>Thinking again, I could have been wrong in my earlier message, it may
>be possible to do it - I'm not familiar enough with the evaluation
>capabilities to say for sure.
>I would look at 'man dhcp-eval', and perhaps try something along the lines 
>of :
>    option routers packet (24,4) ;
>If that doesn't work, if you look back to earlier today, there's a
>thread "Identifying DHCP Relay Agent" which identifies another
>approach you CAN use if your routers are of a limited range of
>addresses (ie you can write a class statement or an if statement for
>each distinct address a router may be at).
>The reason for checking that you aren't using a shared network is
>that in a shared network (which is where you have more than one
>subnet on a physical network), the ip address given by the relay
>agent may not be in the same subnet as the client will be getting a
>lease in. For example, if and were a shared network,
>then the relay agent could be whilst the client gets
> as an address - clearly would not be a suitable
>address for the client to try and use as it's default router.

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