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ip helper-address behavior

 
 
Terry D
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      02-12-2007
If I have multiple DHCP servers, and in turn, multiple ip helper-
address lines in my router, how does the router handle this? Does it
round robin the requests between the two entries?

 
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headsetadapter.com
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      02-12-2007
It sends it to all servers. When we studied our network, we noticed that
routers convert DHCP request to unicast, and sends to all configured DHCP
servers. So, you cannot control which server will accept and give IP
address.

Good luck,

Mike
------
Cisco IP Phone PC Headset Adapters
www.ciscoheadsetadapter.com


"Terry D" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> If I have multiple DHCP servers, and in turn, multiple ip helper-
> address lines in my router, how does the router handle this? Does it
> round robin the requests between the two entries?
>



 
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Terry D
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      02-12-2007
On Feb 12, 2:11 pm, "headsetadapter.com" <(E-Mail Removed)> wrote:
> It sends it to all servers. When we studied our network, we noticed that
> routers convert DHCP request to unicast, and sends to all configured DHCP
> servers. So, you cannot control which server will accept and give IP
> address.
>
> Good luck,
>
> Mike
> ------
> Cisco IP Phone PC Headset Adapterswww.ciscoheadsetadapter.com
>
> "Terry D" <(E-Mail Removed)> wrote in message
>
> news:(E-Mail Removed) oups.com...
>
> > If I have multiple DHCP servers, and in turn, multiple ip helper-
> > address lines in my router, how does the router handle this? Does it
> > round robin the requests between the two entries?


Interesting. What's best practice? So, both servers could respond
with an ip address but the client only accepts one? Isn't this
wasteful?

 
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Gabriele Beltrame
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      02-13-2007
Hi,

the IP "refused" by the client is not wasted; actually only one IP in the
pool is used.

For High-Availability of DHCP serve I suggest you to use a cluster or the
classical 80/20 config.

Regards,
Gabriele

"Terry D" <(E-Mail Removed)> ha scritto nel messaggio
news:(E-Mail Removed) ups.com...
> On Feb 12, 2:11 pm, "headsetadapter.com" <(E-Mail Removed)> wrote:
> Interesting. What's best practice? So, both servers could respond
> with an ip address but the client only accepts one? Isn't this
> wasteful?
>



 
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Sam Wilson
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Posts: n/a
 
      02-13-2007
In article <(E-Mail Removed). com>,
"Terry D" <(E-Mail Removed)> wrote:

> On Feb 12, 2:11 pm, "headsetadapter.com" <(E-Mail Removed)> wrote:
>
> > "Terry D" <(E-Mail Removed)> wrote in message
> >
> > news:(E-Mail Removed) oups.com...
> >
> > > If I have multiple DHCP servers, and in turn, multiple ip helper-
> > > address lines in my router, how does the router handle this? Does it
> > > round robin the requests between the two entries?

> >
> > It sends it to all servers. When we studied our network, we noticed that
> > routers convert DHCP request to unicast, and sends to all configured DHCP
> > servers. So, you cannot control which server will accept and give IP
> > address.

>
> Interesting. What's best practice? So, both servers could respond
> with an ip address but the client only accepts one? Isn't this
> wasteful?


It's handled by the protocol: client broadcasts request, servers make
offers, client broadcasts acceptance of one of the offers (it's
broadcast so other servers know they've been declined), accepted server
ACKs (or NAKs and the whole thing starts again). So only one address is
used per client.

If you're using dynamic pools then you have to make sure your servers'
pools don't overlap or that they have a way of synchronising their pools
(rare). With statically assigned addresses it doesn't matter - multiple
servers are just for redundancy.

Sam
 
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Thrill5
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Posts: n/a
 
      02-14-2007

"Sam Wilson" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> In article <(E-Mail Removed). com>,
> "Terry D" <(E-Mail Removed)> wrote:
>
>> On Feb 12, 2:11 pm, "headsetadapter.com" <(E-Mail Removed)> wrote:
>>
>> > "Terry D" <(E-Mail Removed)> wrote in message
>> >
>> > news:(E-Mail Removed) oups.com...
>> >
>> > > If I have multiple DHCP servers, and in turn, multiple ip helper-
>> > > address lines in my router, how does the router handle this? Does it
>> > > round robin the requests between the two entries?
>> >
>> > It sends it to all servers. When we studied our network, we noticed
>> > that
>> > routers convert DHCP request to unicast, and sends to all configured
>> > DHCP
>> > servers. So, you cannot control which server will accept and give IP
>> > address.

>>
>> Interesting. What's best practice? So, both servers could respond
>> with an ip address but the client only accepts one? Isn't this
>> wasteful?

>
> It's handled by the protocol: client broadcasts request, servers make
> offers, client broadcasts acceptance of one of the offers (it's
> broadcast so other servers know they've been declined), accepted server
> ACKs (or NAKs and the whole thing starts again). So only one address is
> used per client.
>
> If you're using dynamic pools then you have to make sure your servers'
> pools don't overlap or that they have a way of synchronising their pools
> (rare). With statically assigned addresses it doesn't matter - multiple
> servers are just for redundancy.
>
> Sam


You can also purchase a DHCP system that offers redundancy without having to
use multiple pools. Many of the IP address management systems offer this
capability.

Scott


 
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