Cisco 1841 Conncting to Sonicfirewall Ts170

Unanswered Question
Nov 6th, 2009

Problem :

I am unable to communicate with sonicwall firwall using IP and reason is when I assign the IP address on fa0/0 on router its overlaps with dialer interface ( conecting to internet).

I can't assign public IP addresses on fa0/0 on cisco 1841.( assigned by ISP , 6 IP's) Public IP from the same range is already assgined to dailer interface. Both IP's are on same subnet. I get I get the error message saying overlaps with dailer0.

IP need to establish layer 3 connectivity from Sonicwall to internet to build VPN tunnel.

S0 three IPs on a same subnet.

First IP to Cisco 1841( dialero to internet)

Second IP to the Fa0/0 ( connection to Sonicwall)

3 x IP on Sonicwall

VPN------internet---Cisco 1841 ---- Sonicwall-----Switches--------LAN

Can you please confirm if this setup is possible,

Sonicwall should have public IP address.


I have this problem too.
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Giuseppe Larosa Fri, 11/06/2009 - 06:42

Hello Salman,

IRB can help

int dialer0

no ip address

bridge-group 1

int fas070

no ip address

bridge-group 1

interface BVI1

ip address negotiated

but the problem is how the ip address is achieved by dialer interface, probably by PPP negotiation and I'm not sure this can work.

alternate way:

you configure a private ip address on C2941 lan interface

you change sonicwall configuration to use an ip address taken in the same private ip subnet.

but this would lead to remove the sonicwall from the picture that can be another way to do the job specially if you have a firewall feature set on C1841 (like advanced ip services or better)

Hope to help


s.nasheet Fri, 11/06/2009 - 07:11

Hi Giuseppe ,

If I break ( further subnet) my IP range , then do you think ,I can slove the layer three isuse, as the Cisco 1841 will be on a seperate subnet say and then I can still use the other Ip's ( total 6) on seperate subnet?

Do you think this will work ?



Giuseppe Larosa Fri, 11/06/2009 - 07:47

Hello Salman,

you can further perform subnetting getting two point-to-point /30 ip subnets that can host two ip addresses per subnet.

This can work and allows you to perform NAT on sonicwall but the dialer receives parameters from provider so you may be not able to perform further subnetting.

If you would have a separate even if small public ip address block you would be fine.

Hope to help


s.nasheet Fri, 11/06/2009 - 07:55

Good sugesstion, If I ask my ISP to put my dailer0 ( adsl2+ ) interface on /32 subnet and ask them to put other IP's on different subnet then do you think it will work ?

Thanks for your advise and relpies.


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