about concept "group", "shared-network", and "subnet", thanks.

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Apr 20 22:12:45 UTC 2011

Marc Perea wrote:

>Bruce, Andrew, and Simon - thanks, I appreciate your responses. It's 
>_almost_ clear to me!

I'm not so sure !

>If this drawing show up un-mangled, I'll be amazed...
>---------|                         |            |      | 
>          |       | Opn 82
>DHCPD |-- Server Vlan2 --| Switch |-----| Router   |-----| Access
>---------|                         |            |      | L3 relay | 
>| L2 relay
>So yes, what it looks like to me is that I have several subnets that 
>are all in 1 shared network - vlan2. On vlan2, I only see DHCP 
>broadcasted discovers from other vlan2 servers, something I don't 
>want to answer for, hence my lack of authoritative on that subnet. 
>There are actually 4 L3 relay routers, handling from /18 down to /26 
>networks, and all of these unicast DHCP discovers to the dhcpd 
>server host on vlan2.

Then that is **NOT** a shared network. In your diagram (which will be 
mangled in my reply) you show a router between VLAN2 and your client 
subnets - so VLAN2 is **NOT** part of a shared-network with your 
clients (assuming the router is not configured as a bridge).

Further, since you have 4 L3 relay routers (ie 4 routers), I will 
assume that there are no parallel paths and each network is connected 
via just one router. That being the case, networks connected to 
different routers are NOT part of a shared-network.

If on each router, each subnet is connected to a different port 
(whether real or VLAN ID), then those subnets are not part of 

>  >    The "shared-network" is meant to handle a case where you have two
>>or more IP subnets sharing a single broadcast domain (aka VLAN, etc).
>>In Cisco terms, you want a shared-network when-ever you have an
>>"ip address ... secondary" on an interface.
>That is not the case here. So does that mean I should have 0 
>shared-networks and only have subnet declarations?

I think so !
I suspect you can remove all your shared-network declarations and 
things will "just work" automagically.

>I think this explains why we've seen some devices get wrong IPs in 
>some weird ways in the past.

As has already been said, having a shared network tells the DHCP 
server that two or more IP subnets are interchangeable on the same 
physical network. If you don't in fact have a shared-network, then 
you will see clients getting addresses from the wrong subnet.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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