2GB SFP vs 4GB SFP

[Ben Wold]

This seems like a stupid question but I cannot find reference to it in the SAN Design Guide nor product spec pages.

If I use a 4GB swiths with a 4GB SFP, can I connect to a 2GB HBA? I have no idea if the SFPs are backward compatible.

[Mark Poeschl_2 · Accepted solution]

Absolutely - no issues at all. The switch port will autonegotiate down to whatever speed the HBA supports. The only problem would come up if you had altered the switch port's settings to disable autonegotiation. If you're not a fan of autonegotiation you can also hard-set the switch port's speed to 1, 2 or 4 Gbps.

[SAKET_5]

Ben,

with the majority of 4Gb SAN Switches, there remains an issue with the autonegotiation deamon running on the switch. Leaving the port to autonegotiate is known to cause all sorts of hassles. I do not have a copy of the exact error message handy right now, but autonegotation atleast on SilkWorm 4100 (a 4Gb SAN Switch) also generates a large number of erroneous messages - some have also been to cause grief to SAN based backup solutions.

I have seen it myself that an alpha server with the old model 1Gb KGPSA-CY FCA when attached to a 4Gb switch even failed to boot as auto-negotiation did not complete. Yes the tru64 unix cluster in this case boots off the SAN storage.

Hard code the port speed on your switch end is what I would suggest to you.

[Ben Wold]

Thanks for the info.

[Ben Wold]

Closed