I've worked on a network upgrading from Netgear M4100 to M4300 switches, with the intent to upgrade from 1000BASE-SX SFP modules to 10GBASE-SR SFP+ modules in a second step since the M4300 are SFP/SFP+ slots.

All (that is most) links initially were left at 1G using the SFP modules from the M4100 until we'd swap them out for to SFP+ modules. However even after the fact the remained at 1G even though we previously tested if the old fibers would still support 10GBASE-SR.)

However even after changing to 10GBASE-SR modules most links remained on 1G speed:

(m4300) #show fiber-ports optics-info 1/0/5

                           Link   Link                                    Nominal
                          Length Length                                    Bit                                       Possible
                           50um  62.5um                                   Rate                                        Speed
Port        Vendor Name    [m]    [m]    Serial Number    Part Number     [Mbps]  Rev     Compliance     Supported   Detected
-------- ---------------- ------ ------ ---------------- ---------------- ------- ---- ---------------- ------------ ---------
1/0/5    FS               30     15     XXXXXXXXXXXX      SFP-10GSR-85       10300      10GBase-SR               Yes       1G

(m4300) #show interfaces status 1/0/5

                                         Link    Physical    Physical    Media               Flow Control
Port       Name                          State   Mode        Status      Type                Status
---------  ----------------------------  ------  ----------  ----------  ------------------  ------------
1/0/5                                    Up      Auto        1000 Full   10GBase-SR          Inactive

Lesson learned: 10GBASE-SR modules can be dual-rate

Although I knew many old 2G modules for FiberChannel supported 1G ethernet I didn't knew that at least some 10GBASE-SR modules would also be dual-rate.

On the positive side one can swap out SFP for SFP+ modules on one end of a link, wait for the other end and still get a working connection albeit at 1G before swapping the module on the other end.

Finding the cause and a fix

Shutting down the port and removing / re-inserting the SFP modules on both ends didn't make any difference.

However it dawned on me when I compared a port configuraton "stuck" on 1G against an empty (default) configration:

(m4300) #show running-config interface 1/0/5

!Current Configuration:
!
interface  1/0/5
auto-negotiate
speed auto 1000 full-duplex
[...]
exit

The default (empty) config of unused ports didn't have these lines and didn't appear on the links that were newly installed and thus at 10G.

What eventually fixed the "negotiated" speed was:

configure
interface 1/0/5
no auto-negotiate
speed auto 10G full-duplex
exit

If both ends are M4300's this needs to be "undone" on both sides of the link.

Afterwards the link came back up at 10G and both config lines disappeared from the running-config. This indicates that "no auto-negotiate" and "speed 10G full-duplex" were in fact the default config.

Why did this even happen?

I wanted to know what caused this to even happen so I tried to reproduce the situation:

• It was definitely not a human being, that I was pretty certain, it was not documented in the configuration docs.
• Simply inserting a 1000BASE-SX module in an SFP+ slot wouldn't add the 2 lines.
• Connecting and successfully bringing up the link on 1G however did.

The 2 lines remain even after removing the SFP module and eventually upgrading to SFP+.

Conclusion

If you upgrade from 1G to 10G on M4300 while using the same SFP+ slot, don't forget to undo the auto-added auto-negotiate and speed lines to get full speed on the SFP+ slots.