Choosing appropriate SFP's

[jeffakatz]

My inherited network has a Cisco Catalyst 4506 with a WS-X4124FX–MT fiber card that connects to twelve Cisco 2950 switches over 62.5 micron multimode fiber at 100 Mbps. I do not know my run lengths (or even where the conduits run), but the furthest switches are well over a thousand feet from the server room. I am looking to hire someone with the appropriate test equipment to provide this information soon.

We are looking at upgrading the main switch to a Catalyst WS-C4507+E with two WS-X4712-SFP+E cards and the closets to Cisco 2960S-48TD-L switches. Assuming this is a reasonable move, my question is about choosing the appropriate SFP’s for our current and future needs.

I am aware that 62.5 micron multimode fiber is the least favorable for extended lengths, but I will not be in a position to replace it for at least a year. If I purchase 10 Gbps modules, like the SFP-10G-SR or SFP-10G-LRM, can they “throttle down”, either automatically or by setting a parameter, to communicate at slower speeds over distances that exceed their 10 Gbps maximum link lengths on multimode fiber? I would prefer to leverage my speed options rather than buying slower SFP’s like the 1000BASE-SX or 1000BASE-LX/LH now, then replacing them when I upgrade the fiber. Cisco does publish some white papers regarding long distance (up to 2 km) deployment over multimode fiber using their GLC-LH-SM SFP and mode conditioning patch cords.

Thanks in advance for your recommendations.

[Reza Sharifi]

Hi,

Unfortunately, you can't throttle down the speed on the 10Gig SFPs or X2 interfaces. You would need to buy the Gig optics and replace them with 10Gig later. If your distance is over a thousand feet from the server room, you will not be able to use for example GLC-SX-MM, over 62.5 fiber, because the max distance cable for this optic is 722ft.

You may want to study the cost difference between using different optics (1Gig now and 10Gig later) and replacing your fiber by putting Single-mode or MM over 50 micron using OM3 or OM4. If you use OM3 over MM, the distance is about 300m and for OM4 it is about 400m.

Couple of links for references:

http://www.cisco.com/en/US/prod/collateral/modules/ps5455/data_sheet_c78-455693.html

http://www.cisco.com/en/US/docs/routers/7200/install_and_upgrade/gbic_sfp_modules_install/5067g.html

HTH