Category / Nexus

1GB SFP in SFP+ slots on Cisco Nexus 5000 series switches March 30, 2012 at 10:00


One very common thing that I keep getting asked is “Why doesn’t this 1 gigabit SFP module work in this 5548 series Nexus switch? The specs say it will!”

It’s actually something that’s really really simple to fix.


1Gbit SFP in SFP+ slot, “show interface brief” displays the following for the port in question:

Eth1/11       1      eth  access down    SFP validation failed       10G(D) —


The reason this is happening is for one of two reasons:

1.  You’re on an older 5020, and you’re plugging into a port numbered 17 or higher.  In which case, find a lower numbered port to use and see step 2.  If you have no other ports from 1-16, you need to find another switch to plug into. 🙂  Older Nexus 5000 series can only support SFP in SFP+ in the first 16 slots.  Don’t ask me why, it’s just how they’re built.  This is fixed in the newer switches, including the 5548.

2. You’re on any given 5000 series switch, and the port doesn’t know it’s supposed to be a 1Gbit port.  The ports can tell they have an module plugged into them, and they’re looking for certain validation bits from the module to say if it’s a SFP or an SFP+ module.  They can’t, however, autodetect what sort is actually there, all they know is that it’s not passing its validation check.


The fix for this is pretty easy.  All you have to do is tell the switch what the speed of the SFP it’s got plugged in is running at.


#conf t
Enter configuration commands, one per line. End with CNTL/Z.
# int Ethernet1/11
# speed 1000

#show interfaces brief

Eth1/11       1      eth  access down    Link not connected         1000(D) —

The other side isn’t up yet, but at least the switch can see that the SFP is valid and good.  Proceed with configuring the switch normally.

NX-OS is something that I’ve been spending a  bunch of time working with lately, there will be more configuration tidbits and tips/tricks coming down the pipe in the future.