Wonder if anyone knows if this can be done if so how is it done!.

We have Two 2955's. Wan 1 on each end connects over a long range Microwave radio unit which can be thought of as a long piece of CAT 5 cable. WAN 2 we intend to connect via a VPN over ADSL in case that fails. At end One we have a LAN address range of 10.20.30.XXX at the other end we need to use the same address range i.e. 10.20.30.xxx Now AIUI we normally when doing that have a different address range at one end so one end perhaps would be 10.20.40.xxx

Bit complex to explain quite why we need this but any way of making it all with the same address ranges on each end?. We only need to run Two items of equipment over this system and can restrict the number of .xxx's but as mentioned they must be the same First 3 octets at each end.

If that cannot be done could we use say another simple router at one end to change the address with NAT so,

End A LAN is 10.20.30.xxx via 2955 then radio link to other end 2955 LAN address end B is 10.20.40.xxx if that has to be a different range, then could we get another router to do a 10.20.40.xxx to 10.20.30.xxx conversion?.

Any help gratefully appreciated:)

thanks..

if your microwave thing is really transport of ethernet-frames through the air (a bridge), then you dont have to connect the Ethernet-cable to the WAN of the routers, but to one of the lan ports, no additional setup is neccessary.
Then you have what you desire (but all traffic will go through the bridge, regardless if it makes sense or not)

It gets more complicated regarding the backup solution. If you want to do that using VPN you need two different subnets, and, this is not compatible with the setup above.
That could be done with two /25 subnets.
Location A: 192.168.1.0/25 ( host Adresses from 192.168.1.1 to 192.168.1.127)
Location B: 192.168.1.128/25 (host addresses from 192.168.1.129 to 192.168.1.254)
This will work with VPN, but, will conflict with your bridge.
But, if you use the bridge for a VPN connection between the 2 subnets, that will work.
Connect the two ends of the bridge to one of the WAN ports, assign static external IPs to the Routers WAN ports, and, create a VPN between the two locations.
Connect your DSL to WAN2, and, also create a VPN between these location using WAN2.

you then can use the 295x VPN trunk options to normally use the VPN with WAN1 (where your microwave bridge is connected) or, if this is down, use the VPN tunnel through the ADSL on WAN2.

But this is not the "same lan", these are two lans, but, they have identical 3 first octets - which is what you asked for.

Thanks for that!. I think i Have the gist of that!. Just to clarify if say I needed to have communication between Two units say on IP address 192.168.1.123 which can and do run across the "bridge" fine, it is totally transparent. Can that run across the backup circuit when say the bridge microwave link is down?. The unit is question is driven by a server the program in that has a fixed IP address to the unit in question and Two way comms are required as such between the driven unit and server..

if the Bridge connection is down, it might take a little until the VPN connection is online. That might lead to a temporary packet loss, But if the application can handle that, it should work the same way as the bridge.
If you use the bridge without a VPN, anything which uses Ethernet frames can be sent trough the connection. If you use VPN over the Bridge or ADSL, only TCP/UDP/Ping etc. packets can be used for communication and broadcasts will not reach the other side.
The latency of the VPN over ADSL is most likely higher then with the bridge. My ping times are in a range from 30ms (SDSL 2048) to 70ms (ADSL 1024). So, if you application expects answers from the other side within <100ms you have a problem. However, I dont know the current latency over your bridge.
The Bandwidth of the VPN over ADSL is limited to the line speed. If you have an ADSL and you use bidirectional communication, dont expect more throughput then 80% of the upload speed of the connection.

When you say "..... communication between Two units say on IP address 192.168.1.123 .....":
Of course, the IP Adresses must be different, regardless if you have one subnet with a 24 Bit netmask or 2 Subnets with a 25 Bit Netmask. if unit A has 192.168.1.123, the device on the other side must have an IP like 192.168.1.135.