Technical Tip: Auto-negotiation speed for SFP fibre ports on 90G/91G series

FortiGate-90G/91G Series have 2x 10/5/2.5/GE RJ45 or 10GE/GE SFP+/SFP shared media ports.

If FortiGate is linked up with SFP fibre ports to a switch 1000-auto, the speed options on the FortiGate are as follows:

FGT91G-2 (wan1) # set speed
1000auto 1000M auto-negotiation.
2500auto 2500M auto-negotiation.
5000auto 5000M auto-negotiation.
10000auto 10G auto-negotiation.

If RJ45 or other cable is used for connection to a 1G speed switch port, the link is UP at any speed. If SFP_fibre is used to connect to a 1G speed FortiGate SFP port, the link will be DOWN at 10000auto speed.

FGT91G-2 # show system interface wan1 | grep speed
set speed 1000auto

FGT91G-2 # get system interface physical wan1
== [onboard]
==[wan1]
                 mode: dhcp
                 ip: 0.0.0.0 0.0.0.0
                 ipv6: ::/0
                 status: up
                 speed: 1000Mbps (Duplex: full)<<===port up at 1000auto
                 FEC: none
                 FEC_cap: none

If the speed port is changed to 10000auto, the status becomes down:

FGT91G-2 # show system interface wan1 | grep speed <- Change to 10000auto.
set speed 10000auto

FGT91G-2 # get system interface physical wan1
== [onboard]
==[wan1]
                  mode: dhcp
                  ip: 0.0.0.0 0.0.0.0
                  ipv6: ::/0
                  status: down <- Port change to down, Status is Down.
                  speed: n/a
                  FEC: none
                  FEC_cap: none

For SFP fibre, when 'set speed 10000auto' is in place, it can only link up with 10G speed. When 'set speed 5000auto/2500auto/1000auto' is configured, it can only link up with 1G speed.

This is a hardware limitation because 90G/91G SFP ports do not support Auto-negotiation.

Note:

Check that both ends of the fibre link are manually set to the same speed, as mismatched configurations (example: 10G on FortiGate and 1G on the switch) will result in link failure.

Starting with v7.6.1, the 1000auto speed option has been removed. During the upgrade, any SFP+ port configured for 1000auto will be changed to 1000full. This may cause the link to go down if the connected device operates at a different speed.

Related articles:

Technical Tip: Speed option 1000full not available on certain ports 

Removed speed setting affects SFP+ interfaces after upgrade