SFPs and Break out cables, speed and MTU settings

Question

We have a 524D switch, and using a 40GB breakout cable to 10GB. The FortiSwitches are managed through the Fortigate. We then have a transceiver to be plugged in to an HPE Gen 10 Server. The documentation mentions the following: "Auto-speed detection is supported on 1/10G ports, but not on higher speed ports (such as 40G)". What does this mean for the 4 x 10GB speed ports? Can they be set to Auto 10GB? The port speed on the present Cisco transceivers in the HPE Gen 10 servers are already set to 10GB auto. Also I am not understanding what the speed setting called Auto-Module is used for? We are running version 6.4 version of firmware.

And one more question regarding MTUs. If we look at the settings on the switch this is what we see. " The VLANS are set to an MTU of 1500, but the ports themselves are set to 9198". This is confusing. Does one. overide the other? Thank you

akileshc · Accepted Answer

Hello,

1. "Auto-speed detection is supported on 1/10G ports, but not on higher speed ports (such as 40G)". What does this mean for the 4 x 10GB speed ports? Can they be set to Auto 10GB?

Answer: Enabling auto-module speed detection, the system reads data from the QSA/QSFP module and adjusts the port speed to the module's maximum claimed speed.

If the system meets an error when reading from the QSA/QSFP module, it returns to the default speed (the default value is platform-specific).

Autospeed detection is available on 1G and 10G ports, but not on higher speed ports (such as 40G)

Refer to the below link for more details about config and verification: https://docs.fortinet.com/document/fortiswitch/6.4.6/administration-guide/861224/physical-port-settings#Auto-mod

2. If we look at the settings on the switch this is what we see. " The VLANS are set to an MTU of 1500, but the ports themselves are set to 9198". This is confusing. Does one. override the other?

Answer: The MTU size of the VLAN interface is directly proportional to the MTU size of the parent/associated interface.

As an example:
Create a 'vlan test' VLAN interface on physical interface 'Port1'.

If the 'port1' interface MTU is 9198, you can adjust the 'vlan test' MTU to be between 68 - 9198.

However, if the MTU of the 'port1' interface is 1500 and you try to set the MTU of the 'vlan test' to a value greater than 1500, you will receive the following error:

# config system interface
(interface)edit vlan_test
(vlan_test)set mtu-override enable
(vlan_test)set mtu 9170

MTU size not valid. Should be in the range of 68 - 1500.
node_check_object fail! for mtu 9170

value parse error before '9170'
Command fail. Return code -2
end

I hope this clears up certain doubts.

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