FortiSwitch
FortiSwitch: secure, simple and scalable Ethernet solutions
sachitdas_FTNT
Article Id 204448
Description This article describes the recommended port speed configuration when using copper SFP module 1000-Base-T, and explains the network impact if the recommended speed settings are not used.
Scope FortiSwitch version 6.x and above.
Solution

When using the 1000-Base-T RJ45 copper SFP module, the port shows as 'up' or online even when there is no UTP (Unshielded Twisted Pair) cable connected to the Transceivers.

 

The 1000-Base-T RJ45 copper SFP module is a particular module which requires the use of 1000auto speed configuration.

 

Using other speed configs like 1000full or auto-module may cause the link to not come online or could cause the issue in the example below.

 

This example assumes the following setup:

- Laptop1 using port43 through FortiSwitch FSW1 (port47, 48 in LACP).

- FortiSwitch FSW2 using port43 to connect to Laptop2.

- Port47 and port48 are in LACP, using a 1000-Base-T copper SFP module. The default port speed setting is auto-module.

 

Upon disconnecting the RJ-45 cable either on FSW1 or FSW 2 on port47 or port48, the traffic between the laptops is lost for 20-30 seconds.

 

When the SFP module is disconnected, less than 2 seconds of drop are visible.

 

This occurs because, according to the switch, the mac layer port is still up when a cable is disconnected. Therefore, the LACP trunk relies on LACP protocol to detect when a member port is down instead of when a physical port is down, meaning the process takes longer.

 

Solution:

Change the speed on port47 and port48 on both switches to 1000auto.

 

Run the following command to do this:

 

# config switch physical-port

    edit port#

        set speed 1000auto

    end

 

Note that auto-module is not auto-negotiation. 1000auto simply enables auto-negotiation.

Additionally, auto-module means disabling the auto-negotiation and auto-detect speed based on the module, meaning it will not negotiate speed with the remote side.