configuring IP helper address

Answered Question
Feb 25th, 2009
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We Have a dhcp pool configured in our core router...


with multiple subinterfaces configured on the ethernet interface for each diff. vlan


however our Windows team wants some

servers to go to a diff dhcp server via

the helper address



how could i configure this since the ip helper address is usually configured under the ethernet port that the switch is connected to?



for example here is a portion of our router config


p dhcp pool POB

network 10.1.12.0 255.255.254.0

default-router 10.1.12.1

dns-server 192.168.119.9.6


p dhcp pool C

network 10.1.14.0 255.255.254.0

default-router 10.1.14.1

dns-server 192.168.119.9


FastEthernet0/0.12 10.1.16.3 YES manual up up


FastEthernet0/0.14 10.1.18.3 YES manual up up


Correct Answer by Jon Marshall about 8 years 2 months ago

Should work fine as ip helper-address command can be configured under the subinterface as well as the interface level.


One other thing. Why are you using DHCP for servers ? is it for the management interface on the servers ?


Jon

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Edison Ortiz Wed, 02/25/2009 - 08:41
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I don't see how you can force (from the router side) clients to use one DHCP server over another.


When the client boots up, it broadcasts a DHCP request and the first DHCP server that replies will service that client.


Having the ip helper-address under the interface in addition to having a DHCP server local to that segment, adds complexity into the DHCP server selection.


I'm not well-versed on the DHCP server side of things but from what I understand, you can accomplish your design with having multiple Windows DHCP servers sharing a single DHCP database. I'm sure there are other DHCP servers that do the same but Cisco IOS DHCP Server isn't one of them.


HTH,


__


Edison.

Jon Marshall Wed, 02/25/2009 - 08:50
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Richard


I agree with Edison on this. Having multiple DHCP servers managed by different groups of people is a recipe for confusion. Technically, as Edison says, if you have a local DHCP server this will answer before any remote DHCP server so it would be very difficult if not impossible to do it.


If you have Windows Servers that can act as DHCP servers i would suggest migrating all DHCP responsibilities to those servers and let the router do what it does best ie. route packets across the network.


Jon

nygenxny123 Wed, 02/25/2009 - 09:04
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Great input..



Just as a guess..


If i configured an entirely new vlan

and subinterface ...


dedicated those server/switchports to that vlan


and configured the ip helper under

the new subinterface


would this still not work due to the broadcast issue?

Edison Ortiz Wed, 02/25/2009 - 09:08
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If you aren't creating a DHCP scope in the router for that Vlan, then your plan may work.


__


Edison.

lamav Wed, 02/25/2009 - 09:36
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"The router running the DHCP server will be a candidate to reply on DHCP requests."


Im not sure about that. The router is doing inter-vlan routing (router on a stick), and each of those vlans is isolated from each other. Moreover, the DHCP pool that is configured on the router will respond to clients that sit on that vlan/subnet. The IOS DHCP server is not over-arching and does not respond to all requests on all subnets. This is why separate DHCP instances are configured on the route r-- one for eahc vlan.


So, given the question, it seems feasible (on paper, at least), that a new subinterface can be created on the router to support a separate vlan/subnet and have those clients broadcast their L2 DHCP Discovery packets, which can then be forwarded via a helper address.


As I said, it seems feasible on paper -- in theory, but this would have to be tested in a lab, of course.


Lastly, I do agree with both Jon and Edison that you should offload DHCP responsibilities to dedicated Windows/UNIX servers and leave the router to do what it does best.


Thanks

Correct Answer
Jon Marshall Wed, 02/25/2009 - 09:27
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Should work fine as ip helper-address command can be configured under the subinterface as well as the interface level.


One other thing. Why are you using DHCP for servers ? is it for the management interface on the servers ?


Jon

nygenxny123 Wed, 02/25/2009 - 09:30
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This is a installation set up.


They are remotely going to install images

on the servers from from a remote location..


At least thats what they tell me


:-)

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