FortiSwitch
FortiSwitch: secure, simple and scalable Ethernet solutions
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Article Id 347877
Description This article describes spanning tree protocol (STP) behavior in LAG and MCLAG.
Scope FortiSwitch.
Solution

LAGs and MCLAGs are often preferred for redundancy and higher performance.

 

From the STP perspective LAGs in MCLAG are seen as a single logical interface.

LAGs in MCLAG are considered up if one of the links is UP, this means any changes in the state of any one link of MCLAG or LAG will not cause an STP change unless all LAG/MCLAG links come down.

 

The MCLAG client sees the two MCLAG peers as a single switch participating in STP.

 

The example below shows 4 interconnected switches.

Switch1 and switch2 are MCLAG peers. Switch3 is an MCLAG client and switch4 is connected to switch3 via regular lag.

From the STP point of view, MCLAG peers are seen as a single switch and uplinks on switch3 and switch4 are treated as single interfaces.

 

Screenshot 2024-10-08 180834.png

 

Related documents: 

MCLAG

STP

Link aggregation groups