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Assigning DHCP via Dial-up to a different subnet than the DHCP relay address

Posted on 2004-04-14
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Last Modified: 2010-04-17
Hello,

Up until now,  dial-in users have been using a local scope set up on a Cisco 2500 Router to assign IP addresses from a range 10.160.114.0, This has been working fine, but does not allow the assigning of other scope options which I now require.

The problem I have is that the DHCP relay on the cisco (ip-helper) is on int e0 with an IP address of 10.160.116.2 and as I understand, the DHCP relay attaches that IP address (int e0) as a marker, and the DHCP server will make the descision on which scope to assign an IP from based on the DHCP relay address, ie 10.160.116.0 range (wich works fine).  Can I without changing the primary int e0 address, trick my DHCP Server (Win NT) into assigning a range from the 10.160.114.0 range.

Any pointer will be gratefully accepted, and thanks for your time.
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Question by:krisleeson
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by:mikebernhardt
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I don't know what kind of interfaces the dial-in users are dialing in on, but if you set up a dialer interface or group-async interface which you then associate with any number of physical interfaces, you can do what you want. You give the new interface an address, which will end up being the default gateway address of the dial in users. Put your DHCP forwarding on the dialer interface and it ought to work.

I like dialer interfaces personally because you have a lot more flexibility, but you may not need that. You associate them by putting "dialer pool-member [some number] on the physical, async or group-async interfaces and "dialer pool [same number] on the dialer interface.

Does that help? I'm trying to find a good example in Cisco's public documentation and I'll add it in if I find it.
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by:mikebernhardt
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To clarify one point in my above answer- you can assign an IP address to the dialer interface or group-async interface in the new IP subnet. I assume that you are currently using ip unnumbered to Eth0?
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by:mikebernhardt
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Well, I haven't found a good example. An important point with a dialer interface is that if your physical interfaces are analog, you have to put "dialer inband" command on the dialer interface.
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by:krisleeson
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Thanks for the quick response, I guess I didn't give you too much information to work with, I think I understand what you mean, but can't seem to implement it, so I'll give you the portion of the config that is bugging me!!

1 Ethernet/IEEE 802.3 interface(s)
1 Serial network interface(s)
16 terminal line(s)
---------------------------------------
interface Ethernet0
 ip address 10.160.116.2 255.255.255.0
 no ip mroute-cache
 no cdp enable
!
interface Serial0
 no ip address
 encapsulation ppp
 no ip mroute-cache
 shutdown
 no fair-queue
!
interface Group-Async1
 ip unnumbered Ethernet0
 ip helper-address 99.1.12.24
 encapsulation ppp
 dialer in-band
 dialer-group 1
 async mode interactive
 peer default ip address dhcp
 no cdp enable
 ppp authentication pap dialuplist
 group-range 1 16

So as you can see there is no dialer interface, only the Group-Async interface, and the only physical interface I can associate the Group-Async1 with wich is Int e0 (10.160.116.2).

Many Thanks in advance....
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by:mikebernhardt
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Have you tried to simply give the group-async interface an ip address of it's own? For example,
ip address 10.160.114.1 255.255.255.0.

Set your DHCP scope so that is the default gateway address of dialin hosts.
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by:krisleeson
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Thanks,

I have tried this, but for some reason the only ip command it will recognise on the Group-Async1 is that of  'ip address negotiated ....'  I have tried removing the 'ip unnumbered ethernet0' first, but still with no joy. Also removing the 'peer default ip address ....' command does not allow that adding of an IP address afterwards

Trying to add the suggested command on int group-async1 as ip address 10.160.114.1 255.255.255.0, brings back the old % Invalid input detected at '^' marker. ('^' points to the the first number in the ip address).

At the end of the day, if I have to, I will change the int e0's ip address to the subnet I want for DHCP, but would be useful to find another way (if there is one)

Sorry to be a pain..... and thanks again.

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mikebernhardt earned 450 total points
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Try something like this then. The idea is that all the layer 3 stuff is put on the dialer interface, which is associated via pool 1 with your async group. You may need to tweak it, but if the basic concept works you'll be on your way:

conf t
interface Group-Async1
 encapsulation ppp
 no cdp enable
 dialer in-band
 dialer-pool-member 1
 async mode interactive
 group-range 1 16

interface dialer 1
 encapsulation ppp
 ip address 10.160.114.1 255.255.255.0
 peer default ip address dhcp
 no cdp enable
 peer default ip address dhcp
 ppp authentication pap dialuplist
 ip helper-address 99.1.12.24
 dialer-group 1

If that doesn't work, I apologize for wasting your time...
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Author Comment

by:krisleeson
Comment Utility
Ok.... Thanks for your persistence, with some tweaking as you recommended I have managed to get it to work, by introducing Dialer 1, giving it an IP address and an ip helper-address.  Then pointing the ip unnumbered on Group-Async1 to Dialer1.  As below:

interface Group-Async1
 ip unnumbered Dialer1
 ip helper-address 99.1.12.24
 encapsulation ppp
 dialer in-band
 async mode interactive
 peer default ip address dhcp
 no cdp enable
 ppp authentication pap dialuplist
 group-range 1 16
!
interface Dialer1
 ip address 10.160.114.2 255.255.255.0
 ip helper-address 99.1.12.24

Thanks again...
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Expert Comment

by:mikebernhardt
Comment Utility
I wonder then if you could have pointed Group-Async 1 at a loopback interface...
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