Regarding option routers

Simon Hobson dhcp1 at
Thu Jul 13 15:44:08 UTC 2006

Vithal Shirodkar wrote:

>Thanks for the detailed response and the note of caution.  I am running some
>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 to
>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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