| Description | This article explains the reason and solution for network instability that occurs when multiple Tier-2 MCLAG-ICL FortiSwitch pairs are connected to a Tier-1 MCLAG-ICL pair when setup in FortiLink mode. |
| Scope | FortiSwitch OS v7.4, v7.6. |
| Solution |
Topology:
Note: this issue is only when the setup is in FortiLink Mode.
Reason:
Example (Pair 1 connection):
When Pair 1 is connected to Tier-1, the following trunk is formed:
Output from FortiSwitch1 / FortiSwitch2
show switch trunk
Issue when Pair 2 is connected:
Instead of creating a new trunk for Pair 2, the Tier-1 FortiSwitch automatically appends the new ports to the existing trunk, causing loops and instability.
Output from FortiSwitch1 / FortiSwitch2
show switch trunk
This behavior results in both Tier-2 MCLAG-ICL pairs sharing the same trunk, which is not a valid design and leads to network instability.
Solution:
Disconnect the Tier-2 links from Tier-1 and verify that the trunk has been removed. If the trunk remains, delete it manually:
show switch trunk
On the Tier-1 FortiSwitches (FortiSwitch1 / FortiSwitch2), configure separate port groups:
config switch auto-isl-port-group
This ensures that each Tier-2 pair forms its own independent trunk.
After reconnecting the Tier-2 links, verify that separate trunks are created for each pair.
Output from FortiSwitch1 / FortiSwitch2:
show switch trunk
edit "edit tier2-1
set mode lacp-active
set auto-isl 1
set mclag enable
set members "port4" "port5"
next
edit "edit tier2-2
set mode lacp-active
set auto-isl 1
set mclag enable
set members "port6" "port7"
next
end
Note: The trunk name is automatically derived from the auto-isl-port-group name.
For a deeper understanding of FortiLink trunk, see Technical Tip: Understanding FortiLink Trunks and the Parameters auto-isl, fortilink, and isl-fortil....
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