Two DHCP Servers One Wire?

Is it an allowed / supported configuration to have two DHCP server on a
single ethernet segment, each supporting a different class C network? I
certainly have no problems with getting two Class C networks to interoperate
on the same ethernet segment (albeit you will need to route between machines
on different networks).

What I had hoped to do was to have a single DHCP scope serve a historical
class C network, and have a second DHCP scope work only via reservations on
machines being migrated away from the old class C. The new class C will
among other things use a different router for outgoing Internet.

When I tried to do this, machines requiring DHCP are just hanging. So
apparently the two DHCP scopes are not interoperating on the same wire when
one of the two is set to only work with reservations (i.e., all addresses
are excluded).

Yes, of course I could do this with different ethernet segments - one per
class C network. It's just an awful lot of work to do this (it means
putting in new switches in every cubicle and attaching those to additional
uplink RJ-45s, wiring those to different switches in the central wiring
room, etc. I would really really prefer to find some way to automatically
hand out the different network settings to the two different class C
computers while they co exist on the same wire. Is there a practical way
to do this?

--
Will

You can have a couple hundred (not that I would) of them on the same wire as
long as they don't "overlap" in the addresses that they lease-out.


--
Phillip Windell [MCP, MVP, CCNA]
www.wandtv.com


"Will" <westes-(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ...
> Is it an allowed / supported configuration to have two DHCP server on a
> single ethernet segment, each supporting a different class C network? I
> certainly have no problems with getting two Class C networks to
interoperate
> on the same ethernet segment (albeit you will need to route between
machines
> on different networks).
>
> What I had hoped to do was to have a single DHCP scope serve a historical
> class C network, and have a second DHCP scope work only via reservations
on
> machines being migrated away from the old class C. The new class C will
> among other things use a different router for outgoing Internet.
>
> When I tried to do this, machines requiring DHCP are just hanging. So
> apparently the two DHCP scopes are not interoperating on the same wire
when
> one of the two is set to only work with reservations (i.e., all addresses
> are excluded).
>
> Yes, of course I could do this with different ethernet segments - one per
> class C network. It's just an awful lot of work to do this (it means
> putting in new switches in every cubicle and attaching those to additional
> uplink RJ-45s, wiring those to different switches in the central wiring
> room, etc. I would really really prefer to find some way to
automatically
> hand out the different network settings to the two different class C
> computers while they co exist on the same wire. Is there a practical
way
> to do this?
>
> --
> Will
>
>

Aloha Phillip,

We frequently configure clients with two DHCP servers (and non-overlapping
ranges of course) for redundancy and load balancing.

For instance if they have 60 client machines we might set up one DHCP server
to give out 192.168.1.21-1.100 and the other to give out 1.101-1.199. 1.1
is their default gateway and 1.2-1.20 we usually reserve for servers and
devices that need static IP addresses (printers, copiers, WAPs, etc.) The
rest of the scope options would match. Either server has a large enough
range to support the entire LAN by itself or a client can get an address
from either server without any problems.

-Ben-
Ben M. Schorr - MVP
Roland Schorr & Tower
http://www.rolandschorr.com
Microsoft OneNote FAQ: http://www.factplace.com/onenotefaq.htm

> You can have a couple hundred (not that I would) of them on the same
> wire as long as they don't "overlap" in the addresses that they
> lease-out.
>
> "Will" <westes-(E-Mail Removed)> wrote in message
> news:(E-Mail Removed) ...
>
>> Is it an allowed / supported configuration to have two DHCP server on
>> a single ethernet segment, each supporting a different class C
>> network? I certainly have no problems with getting two Class C
>> networks to
>>
> interoperate
>
>> on the same ethernet segment (albeit you will need to route between
>>
> machines
>
>> on different networks).
>>
>> What I had hoped to do was to have a single DHCP scope serve a
>> historical class C network, and have a second DHCP scope work only
>> via reservations
>>
> on
>
>> machines being migrated away from the old class C. The new class C
>> will among other things use a different router for outgoing Internet.
>>
>> When I tried to do this, machines requiring DHCP are just hanging.
>> So apparently the two DHCP scopes are not interoperating on the same
>> wire
>>
> when
>
>> one of the two is set to only work with reservations (i.e., all
>> addresses are excluded).
>>
>> Yes, of course I could do this with different ethernet segments - one
>> per class C network. It's just an awful lot of work to do this (it
>> means putting in new switches in every cubicle and attaching those to
>> additional uplink RJ-45s, wiring those to different switches in the
>> central wiring room, etc. I would really really prefer to find some
>> way to
>>
> automatically
>
>> hand out the different network settings to the two different class C
>> computers while they co exist on the same wire. Is there a
>> practical
>>
> way
>
>> to do this?
>>
>> -- Will
>>

Hey Ben, how's it going?!

I have our TV Station running on a pair of redundant DHCP servers exactly
the way you described. They do 50/50 so each one has enough addresses on
the largest segment to keep the whole mess running. But the smaller segments
might be in trouble if I didn't get the other server back up failry quickly,
those small segments don't have many free address left.

--
Phillip Windell [MCP, MVP, CCNA]
www.wandtv.com


"Ben M. Schorr - MVP" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) .com...
> Aloha Phillip,
>
> We frequently configure clients with two DHCP servers (and non-overlapping
> ranges of course) for redundancy and load balancing.
>
> For instance if they have 60 client machines we might set up one DHCP
server
> to give out 192.168.1.21-1.100 and the other to give out 1.101-1.199. 1.1
> is their default gateway and 1.2-1.20 we usually reserve for servers and
> devices that need static IP addresses (printers, copiers, WAPs, etc.) The
> rest of the scope options would match. Either server has a large enough
> range to support the entire LAN by itself or a client can get an address
> from either server without any problems.
>
> -Ben-
> Ben M. Schorr - MVP
> Roland Schorr & Tower
> http://www.rolandschorr.com
> Microsoft OneNote FAQ: http://www.factplace.com/onenotefaq.htm
>
> > You can have a couple hundred (not that I would) of them on the same
> > wire as long as they don't "overlap" in the addresses that they
> > lease-out.
> >
> > "Will" <westes-(E-Mail Removed)> wrote in message
> > news:(E-Mail Removed) ...
> >
> >> Is it an allowed / supported configuration to have two DHCP server on
> >> a single ethernet segment, each supporting a different class C
> >> network? I certainly have no problems with getting two Class C
> >> networks to
> >>
> > interoperate
> >
> >> on the same ethernet segment (albeit you will need to route between
> >>
> > machines
> >
> >> on different networks).
> >>
> >> What I had hoped to do was to have a single DHCP scope serve a
> >> historical class C network, and have a second DHCP scope work only
> >> via reservations
> >>
> > on
> >
> >> machines being migrated away from the old class C. The new class C
> >> will among other things use a different router for outgoing Internet.
> >>
> >> When I tried to do this, machines requiring DHCP are just hanging.
> >> So apparently the two DHCP scopes are not interoperating on the same
> >> wire
> >>
> > when
> >
> >> one of the two is set to only work with reservations (i.e., all
> >> addresses are excluded).
> >>
> >> Yes, of course I could do this with different ethernet segments - one
> >> per class C network. It's just an awful lot of work to do this (it
> >> means putting in new switches in every cubicle and attaching those to
> >> additional uplink RJ-45s, wiring those to different switches in the
> >> central wiring room, etc. I would really really prefer to find some
> >> way to
> >>
> > automatically
> >
> >> hand out the different network settings to the two different class C
> >> computers while they co exist on the same wire. Is there a
> >> practical
> >>
> > way
> >
> >> to do this?
> >>
> >> -- Will
> >>
>
>

So a bit more detail then:

Class C 172.16.1.x has a DHCP scope with 60 available IPs
Class C 172.16.3.x has a DHCP scope with 0 available IPs, and two
reservations (which per another thread I started recently, is apparently a
legal configuration)

Both Class C networks share the same wire, but their DHCP options should be
pointing them to different default routers.

What I would expect to happen here is that any machine with a reservation on
the 172.16.3.x network would get an address per the reservation. Any
machine with no reservation on either network would grab an available IP
from 172.16.1.x.

What happens instead is that as soon as both scopes are made available,
computers with no reservation simply hang.

Perhaps there is a requirement for both scopes that overlap the same
ethernet to be supported on a single server?

--
Will



"Phillip Windell" <@.> wrote in message
news:(E-Mail Removed)...
> You can have a couple hundred (not that I would) of them on the same wire
as
> long as they don't "overlap" in the addresses that they lease-out.
>
>
> --
> Phillip Windell [MCP, MVP, CCNA]
> www.wandtv.com
>
>
> "Will" <westes-(E-Mail Removed)> wrote in message
> news:(E-Mail Removed) ...
> > Is it an allowed / supported configuration to have two DHCP server on a
> > single ethernet segment, each supporting a different class C network?
I
> > certainly have no problems with getting two Class C networks to
> interoperate
> > on the same ethernet segment (albeit you will need to route between
> machines
> > on different networks).
> >
> > What I had hoped to do was to have a single DHCP scope serve a
historical
> > class C network, and have a second DHCP scope work only via reservations
> on
> > machines being migrated away from the old class C. The new class C
will
> > among other things use a different router for outgoing Internet.
> >
> > When I tried to do this, machines requiring DHCP are just hanging. So
> > apparently the two DHCP scopes are not interoperating on the same wire
> when
> > one of the two is set to only work with reservations (i.e., all
addresses
> > are excluded).
> >
> > Yes, of course I could do this with different ethernet segments - one
per
> > class C network. It's just an awful lot of work to do this (it means
> > putting in new switches in every cubicle and attaching those to
additional
> > uplink RJ-45s, wiring those to different switches in the central wiring
> > room, etc. I would really really prefer to find some way to
> automatically
> > hand out the different network settings to the two different class C
> > computers while they co exist on the same wire. Is there a practical
> way
> > to do this?
> >
> > --
> > Will
> >
> >
>
>

"Will" <westes-(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ...
> Both Class C networks share the same wire, but their DHCP options should
be
> pointing them to different default routers.

You need to get them on a different wire unless you are VLANing.
What you are describing is a Multi-Net. In my opinion they are a nightmare
and have pretty much had their justification eliminated by the invention of
VLANs.

> Perhaps there is a requirement for both scopes that overlap the same
> ethernet to be supported on a single server?

Scopes should never overlap. Servers can overlap (identical Scopes,
different Exclusions),...Scopes on the same server together should not.

--
Phillip Windell [MCP, MVP, CCNA]
www.wandtv.com