Spanningtree Problem Using FortiLink and Cisco Switches

[Samsy21]

Hello Forum,

im trying to setup FortiLink in this setup:

Fortigate <-> Cisco Switch <-> FortiSwitch

I get the FortiSwitch up and running, connecting through the Cisco Switch with native VLAN set to the FortiSwitch VLAN.

The Switch is fully manageable through the FortiGate -- Great Success!!

But i facing the issue if Im connecting a second interface to the FortiSwitch and Cisco Switch.

I wanted to setup this connection for redundancy so the topology would look like this:

If im connecting the second Port of the Portchannel to the FortiSwitch, the link is starting flapping on the cisco site..

Is there maybe a problem with spanningtree oder port-profiles on the FortiSite?

Does anybody of you had this problem earlier?

We have to deploy this enviroment, because we cant change our distribution switch right now.

Thank you!

[Anthony_E]

Hello,

Thank you for using the Community Forum. I will seek to get you an answer or help. We will reply to this thread with an update as soon as possible.

Thanks,

[Anthony_E]

Hello,

To address spanning tree problems when using FortiLink with Cisco switches, follow these steps:

1. Check Protocol Compatibility:

   • Ensure that the spanning tree protocols used on both FortiSwitch and Cisco switches are compatible. FortiSwitch typically uses MSTP, while Cisco switches may use RPVST+. This can lead to convergence issues.

2. Configuration Adjustments:

   • On the Cisco switch, configure the spanning tree mode to match the FortiSwitch configuration. For example, if using MSTP on FortiSwitch, consider configuring MSTP on the Cisco switch as well.
   • Ensure VLAN priorities are set correctly. If the Cisco switch should be the root bridge, its VLAN 1 STP priority must be higher than other VLANs.

3. Verify Port Settings:

   • On FortiSwitch, ensure that the ports connected to the Cisco switch have the correct VLANs allowed and that the edge-port setting is disabled if not directly connected to end devices.
   • Enable RPVST+ on the FortiSwitch port if necessary.

4. Monitor and Troubleshoot:

   • Use diagnostic commands on both FortiSwitch and Cisco switches to monitor STP status and identify any misconfigurations.
   • Commands like show spanning-tree on Cisco and diag stp instance list on FortiSwitch can provide insights into the current STP topology and root bridge status.

By ensuring protocol compatibility and correct configuration, you can resolve most spanning tree issues between FortiLink and Cisco switches.