Extreme SLX-OS Layer 2 Switching Configuration Guide, 20.3.3

General considerations

MCT Peers need to run the same version of SLX-OS.
MCT does not support any variant of Spanning Tree Protocol (STP) on ICL links, cluster client ports, or any VLAN that is part of the cluster. STP is disabled by default.
Note
If you enable STP on MCT nodes, each node acts as an independent (not interconnected) STP switch. This situation results in STP state flap at the node connected to the CCEP port, because it receives two different Bridge Protocol Data Units (BPDU) on that CCEP port
SLX-OS supports dynamic and static LAG between the MCT node and client.
SLX MCT configurations (Layer 2 or Layer 3) do not require the Advance Feature license.

For both MCT peers, the MCT peer IP address must match the source IP address of the peer node.
The source IP and the peer IP should be in the same subnet.
You must configure the same client ID on both MCT peers for the link or CCEP that is connected to the same client.
SLX MCT peers must be physically connected to each other.

If you configure an MCT port channel for multiple VLAN or VE interfaces, use static routes instead of OSPF ECMP.
- As a best practice, configure a static route for MCT peers through the MCT VLAN (VE 10 in this example). A static route has a lower administrative distance than OSPF and places only one route in the routing table.

The LACP fields have the following settings:

Key = MCT_LACP_KEY_BASE (3000) + client_ID

Port ID (16-bit unique value) = 
5-bit (slot value) + 8-bit (port value + 3-bit) (MCT position offset)