The distance limit for longwave SFPs is about 10km, well short of the 25 miles you are looking to achieve. 25 miles is about 40 km, and CWDM optics could do this for you in a point to point fashion. You could use the CWDM mux units with different wavelengths to connection 2 ISLs over the same dark fiber between the 2 sites. DWDM is also an option, and the Cisco ONS 15454 line is one of many DWDM vendors that support fiber channel. Keep in mind that with some non Cisco DWDM solutions you may not be able to run the ISL in a trunk mode. The MDS adds a VSAN header on frames going out ISL ports, and some DWDM gear is not able to pass this larger frame.
Different MDS line cards will provide different BB credits for the ISL. For 40km, at 2GB, with 2K frames, you need about 40 BB credits. If you double the speed to 4GB, you need about 80 BB credits. All the Gen2 and later line cards can easily support this with no additional licensing.
If you do elect to go with DWDM, and they utilize BB spoofing, there is no concern on the RX and TX credits because the Rrdys are generated locally by the DWDM gear, and not the far end MDS port.
Hope this helps,
Mike
