asked on

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

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.

mikebernhardt

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.

mikebernhardt

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?

mikebernhardt

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.

krisleeson

ASKER

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....

mikebernhardt

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.

krisleeson

ASKER

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

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krisleeson

ASKER

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...

mikebernhardt

I wonder then if you could have pointed Group-Async 1 at a loopback interface...