DHCP server assigning addresses on remote subnet

[Guest Scott]

Stupid Question:

 

I have a remote subnet connected via a WAN link. The DHCP server is

installed on the opposite side of the WAN link. The Cisco router is

configured with the IP Helper Address command, to point to the DHCP server's

IP Address, but does not specify an address range/subnet. The DHCP server is

configured for multiple subnets. If the Cisco router is configured only with

the address of the DHCP server, and not any details about which range to

obtain addresses from, how does the DHCP know from which pool to assign

addresses to this remote subnet? Does it use the LAN address from the router

as an index?

[Guest Anthony [MVP]]

Re: DHCP server assigning addresses on remote subnet

 

Scott,

Yes, it knows where the request came from so it can allocate an address from

the correct range. The IP Helper is just forwarding the request, which would

not cross the subnets otherwise,

Anthony,

http://www.airdesk.co.uk

 

 

"Scott" <Scott@discussions.microsoft.com> wrote in message

news:17D37C0E-FED2-4DD3-9DCC-811A8E6F9DFE@microsoft.com...

> Stupid Question:

>

> I have a remote subnet connected via a WAN link. The DHCP server is

> installed on the opposite side of the WAN link. The Cisco router is

> configured with the IP Helper Address command, to point to the DHCP

> server's

> IP Address, but does not specify an address range/subnet. The DHCP server

> is

> configured for multiple subnets. If the Cisco router is configured only

> with

> the address of the DHCP server, and not any details about which range to

> obtain addresses from, how does the DHCP know from which pool to assign

> addresses to this remote subnet? Does it use the LAN address from the

> router

> as an index?

[Guest Scott]

Re: DHCP server assigning addresses on remote subnet

 

So, based on that information. If there were many scopes created, and the LAN

IP Address was configured on the router was accidently set incorrect, it

would request addresses from the wrong scope? (based on the subnet of the

local LAN)?

 

"Anthony [MVP]" wrote:

> Scott,

> Yes, it knows where the request came from so it can allocate an address from

> the correct range. The IP Helper is just forwarding the request, which would

> not cross the subnets otherwise,

> Anthony,

> http://www.airdesk.co.uk

>

>

> "Scott" <Scott@discussions.microsoft.com> wrote in message

> news:17D37C0E-FED2-4DD3-9DCC-811A8E6F9DFE@microsoft.com...

> > Stupid Question:

> >

> > I have a remote subnet connected via a WAN link. The DHCP server is

> > installed on the opposite side of the WAN link. The Cisco router is

> > configured with the IP Helper Address command, to point to the DHCP

> > server's

> > IP Address, but does not specify an address range/subnet. The DHCP server

> > is

> > configured for multiple subnets. If the Cisco router is configured only

> > with

> > the address of the DHCP server, and not any details about which range to

> > obtain addresses from, how does the DHCP know from which pool to assign

> > addresses to this remote subnet? Does it use the LAN address from the

> > router

> > as an index?

>

>

>

[Guest Anthony [MVP]]

Re: DHCP server assigning addresses on remote subnet

 

The IP Helper is the address of the DHCP server, so if that were set

incorrectly there would be no reply,

Anthony,

http://www.airdesk.co.uk

 

 

"Scott" <Scott@discussions.microsoft.com> wrote in message

news:F0C16D18-E551-442D-87D3-15700E7A7819@microsoft.com...

> So, based on that information. If there were many scopes created, and the

> LAN

> IP Address was configured on the router was accidently set incorrect, it

> would request addresses from the wrong scope? (based on the subnet of the

> local LAN)?

>

> "Anthony [MVP]" wrote:

>

>> Scott,

>> Yes, it knows where the request came from so it can allocate an address

>> from

>> the correct range. The IP Helper is just forwarding the request, which

>> would

>> not cross the subnets otherwise,

>> Anthony,

>> http://www.airdesk.co.uk

>>

>>

>> "Scott" <Scott@discussions.microsoft.com> wrote in message

>> news:17D37C0E-FED2-4DD3-9DCC-811A8E6F9DFE@microsoft.com...

>> > Stupid Question:

>> >

>> > I have a remote subnet connected via a WAN link. The DHCP server is

>> > installed on the opposite side of the WAN link. The Cisco router is

>> > configured with the IP Helper Address command, to point to the DHCP

>> > server's

>> > IP Address, but does not specify an address range/subnet. The DHCP

>> > server

>> > is

>> > configured for multiple subnets. If the Cisco router is configured only

>> > with

>> > the address of the DHCP server, and not any details about which range

>> > to

>> > obtain addresses from, how does the DHCP know from which pool to assign

>> > addresses to this remote subnet? Does it use the LAN address from the

>> > router

>> > as an index?

>>

>>

>>

[Guest Scott]

Re: DHCP server assigning addresses on remote subnet

 

Anthony,

 

Sorry, I misstated that. I was just reiterating that the address is assigned

based on the scope that matches the IP subnet of the LAN IP Address for the

FE interface on the router? Just trying to figure out how it knows which

scope to associate with - in the example of many remote subnets and many

address scopes.

 

"Anthony [MVP]" wrote:

> The IP Helper is the address of the DHCP server, so if that were set

> incorrectly there would be no reply,

> Anthony,

> http://www.airdesk.co.uk

>

>

> "Scott" <Scott@discussions.microsoft.com> wrote in message

> news:F0C16D18-E551-442D-87D3-15700E7A7819@microsoft.com...

> > So, based on that information. If there were many scopes created, and the

> > LAN

> > IP Address was configured on the router was accidently set incorrect, it

> > would request addresses from the wrong scope? (based on the subnet of the

> > local LAN)?

> >

> > "Anthony [MVP]" wrote:

> >

> >> Scott,

> >> Yes, it knows where the request came from so it can allocate an address

> >> from

> >> the correct range. The IP Helper is just forwarding the request, which

> >> would

> >> not cross the subnets otherwise,

> >> Anthony,

> >> http://www.airdesk.co.uk

> >>

> >>

> >> "Scott" <Scott@discussions.microsoft.com> wrote in message

> >> news:17D37C0E-FED2-4DD3-9DCC-811A8E6F9DFE@microsoft.com...

> >> > Stupid Question:

> >> >

> >> > I have a remote subnet connected via a WAN link. The DHCP server is

> >> > installed on the opposite side of the WAN link. The Cisco router is

> >> > configured with the IP Helper Address command, to point to the DHCP

> >> > server's

> >> > IP Address, but does not specify an address range/subnet. The DHCP

> >> > server

> >> > is

> >> > configured for multiple subnets. If the Cisco router is configured only

> >> > with

> >> > the address of the DHCP server, and not any details about which range

> >> > to

> >> > obtain addresses from, how does the DHCP know from which pool to assign

> >> > addresses to this remote subnet? Does it use the LAN address from the

> >> > router

> >> > as an index?

> >>

> >>

> >>

>

>

>

[Guest Anthony [MVP]]

Re: DHCP server assigning addresses on remote subnet

 

Yes, based on the router interface it came from,

Anthony,

http://www.airdesk.co.uk

 

 

"Scott" <Scott@discussions.microsoft.com> wrote in message

news:9F1E2F8F-B35F-45DB-8935-9C4276500001@microsoft.com...

> Anthony,

>

> Sorry, I misstated that. I was just reiterating that the address is

> assigned

> based on the scope that matches the IP subnet of the LAN IP Address for

> the

> FE interface on the router? Just trying to figure out how it knows which

> scope to associate with - in the example of many remote subnets and many

> address scopes.

>

> "Anthony [MVP]" wrote:

>

>> The IP Helper is the address of the DHCP server, so if that were set

>> incorrectly there would be no reply,

>> Anthony,

>> http://www.airdesk.co.uk

>>

>>

>> "Scott" <Scott@discussions.microsoft.com> wrote in message

>> news:F0C16D18-E551-442D-87D3-15700E7A7819@microsoft.com...

>> > So, based on that information. If there were many scopes created, and

>> > the

>> > LAN

>> > IP Address was configured on the router was accidently set incorrect,

>> > it

>> > would request addresses from the wrong scope? (based on the subnet of

>> > the

>> > local LAN)?

>> >

>> > "Anthony [MVP]" wrote:

>> >

>> >> Scott,

>> >> Yes, it knows where the request came from so it can allocate an

>> >> address

>> >> from

>> >> the correct range. The IP Helper is just forwarding the request, which

>> >> would

>> >> not cross the subnets otherwise,

>> >> Anthony,

>> >> http://www.airdesk.co.uk

>> >>

>> >>

>> >> "Scott" <Scott@discussions.microsoft.com> wrote in message

>> >> news:17D37C0E-FED2-4DD3-9DCC-811A8E6F9DFE@microsoft.com...

>> >> > Stupid Question:

>> >> >

>> >> > I have a remote subnet connected via a WAN link. The DHCP server is

>> >> > installed on the opposite side of the WAN link. The Cisco router is

>> >> > configured with the IP Helper Address command, to point to the DHCP

>> >> > server's

>> >> > IP Address, but does not specify an address range/subnet. The DHCP

>> >> > server

>> >> > is

>> >> > configured for multiple subnets. If the Cisco router is configured

>> >> > only

>> >> > with

>> >> > the address of the DHCP server, and not any details about which

>> >> > range

>> >> > to

>> >> > obtain addresses from, how does the DHCP know from which pool to

>> >> > assign

>> >> > addresses to this remote subnet? Does it use the LAN address from

>> >> > the

>> >> > router

>> >> > as an index?

>> >>

>> >>

>> >>

>>

>>

>>

[Guest Bill Grant]

Re: DHCP server assigning addresses on remote subnet

 

The DHCP client broadcasts its request on the wire. If the DHCP server is

on that segment it replies with an offer of an IP in the subnet of the

interface on which the request was received.

 

As Anthony pointed out, if the DHCP server is not on the same subnet you

need some device to forward the request to the DHCP server. This forwarded

request will still have the IP address on which the broadcast was received,

so the DHCP server will know which scope to use.

 

If the router has the "wrong" IP as you speculate, it doesn't really

matter. The client needs an IP in the same IP subnet as that router if it is

going to communicate beyond its local segment. If there is in fact no scope

matching that IP, then of course the request will fail.

 

"Anthony [MVP]" <anthony@no-reply.com> wrote in message

news:u4wQIjhtIHA.2292@TK2MSFTNGP03.phx.gbl...

> Yes, based on the router interface it came from,

> Anthony,

> http://www.airdesk.co.uk

>

>

> "Scott" <Scott@discussions.microsoft.com> wrote in message

> news:9F1E2F8F-B35F-45DB-8935-9C4276500001@microsoft.com...

>> Anthony,

>>

>> Sorry, I misstated that. I was just reiterating that the address is

>> assigned

>> based on the scope that matches the IP subnet of the LAN IP Address for

>> the

>> FE interface on the router? Just trying to figure out how it knows which

>> scope to associate with - in the example of many remote subnets and many

>> address scopes.

>>

>> "Anthony [MVP]" wrote:

>>

>>> The IP Helper is the address of the DHCP server, so if that were set

>>> incorrectly there would be no reply,

>>> Anthony,

>>> http://www.airdesk.co.uk

>>>

>>>

>>> "Scott" <Scott@discussions.microsoft.com> wrote in message

>>> news:F0C16D18-E551-442D-87D3-15700E7A7819@microsoft.com...

>>> > So, based on that information. If there were many scopes created, and

>>> > the

>>> > LAN

>>> > IP Address was configured on the router was accidently set incorrect,

>>> > it

>>> > would request addresses from the wrong scope? (based on the subnet of

>>> > the

>>> > local LAN)?

>>> >

>>> > "Anthony [MVP]" wrote:

>>> >

>>> >> Scott,

>>> >> Yes, it knows where the request came from so it can allocate an

>>> >> address

>>> >> from

>>> >> the correct range. The IP Helper is just forwarding the request,

>>> >> which

>>> >> would

>>> >> not cross the subnets otherwise,

>>> >> Anthony,

>>> >> http://www.airdesk.co.uk

>>> >>

>>> >>

>>> >> "Scott" <Scott@discussions.microsoft.com> wrote in message

>>> >> news:17D37C0E-FED2-4DD3-9DCC-811A8E6F9DFE@microsoft.com...

>>> >> > Stupid Question:

>>> >> >

>>> >> > I have a remote subnet connected via a WAN link. The DHCP server is

>>> >> > installed on the opposite side of the WAN link. The Cisco router is

>>> >> > configured with the IP Helper Address command, to point to the DHCP

>>> >> > server's

>>> >> > IP Address, but does not specify an address range/subnet. The DHCP

>>> >> > server

>>> >> > is

>>> >> > configured for multiple subnets. If the Cisco router is configured

>>> >> > only

>>> >> > with

>>> >> > the address of the DHCP server, and not any details about which

>>> >> > range

>>> >> > to

>>> >> > obtain addresses from, how does the DHCP know from which pool to

>>> >> > assign

>>> >> > addresses to this remote subnet? Does it use the LAN address from

>>> >> > the

>>> >> > router

>>> >> > as an index?

>>> >>

>>> >>

>>> >>

>>>

>>>

>>>

>

>