Original Message:
Sent: Jun 18, 2025 10:05 AM
From: thomasbnc
Subject: Spanning tree uplinks on CX 8100 VSX pair
Such a change can be done without interruption. Just remove / re-add lag5 on the core side as MC-LAG interface. While you are doing this, lag6 takes over the traffic from your Agg switches. When done, reconfigure (remove, re-add) lag5 on Agg1 as MC-LAG with one or two link(s) to the MC-LAG on the core. Then configure lag5 (as MC-LAG) on Agg2 and add one of the local links from lag6 (non-MC-LAG) to lag5 (MC-LAG). This should switchover traffic again to lag5. Then decommission lag6 and add the remaining links to the MC-LAG lag5. Spanning-Tree will manage your loops. Just allow enough time after each step to go through the various STP states and let the topology converge.
Original Message:
Sent: Jun 18, 2025 09:17 AM
From: Evan Z
Subject: Spanning tree uplinks on CX 8100 VSX pair
I was hoping to avoid MC-LAG on the Agg 8100s. The previous switches in this place were independent but connected via MC-LAG to the cores.
I will need to schedule an outage to change the uplinks to a MC-LAG with 4 members.
Original Message:
Sent: Jun 18, 2025 06:16 AM
From: vincent.giles
Subject: Spanning tree uplinks on CX 8100 VSX pair
As other community members replied, this topology leads to L2 loop, so spanning-tree will block one path.
Please see the VSX Configuration Best Practices for a topology where a single LAG is used between TOR and AGG, like in this picture (extracted from the new revision of the VSX Best Practices that will be published soon). 
Original Message:
Sent: Jun 17, 2025 04:48 PM
From: Evan Z
Subject: Spanning tree uplinks on CX 8100 VSX pair
I have 2 CX 8100 Aggregation switches configured with VSX. Latest 10.13.1101 code
Upstream is a pair of CX 8360s in VSX as well. Each Agg switch has a LAG to the 2 cores LAG5/6. It is not a MC-LAG on the 8100s
Agg1 switch is working normally but Agg2 switch insists on traversing the inter-switch-link to get back to the cores.
I am getting a STP cost of 802 on the (upstream) MC-LAG vs cost of 800 on Agg1
The cost is only 801 when going over the ISL and other switch's LAG.
I can't set the cost of the ISL. It won't let it change.
I just want this to work as expected.
DC-8100-Agg2# sh span
Spanning tree status : Enabled Protocol: MSTP
MST0
Root ID Priority : 8192
MAC-Address: 02:02:00:00:01:00
Hello time(in seconds):2 Max Age(in seconds):20
Forward Delay(in seconds):15
Bridge ID Priority : 16384
MAC-Address: 02:01:00:00:05:00
Hello time(in seconds):2 Max Age(in seconds):20
Forward Delay(in seconds):15
Port Role State Cost Priority Type BPDU-Tx BPDU-Rx TCN-Tx TCN-Rx
------------ -------------- ---------- -------------- ---------- ---------------- ---------- ---------- ---------- ----------
....
lag6 Alternate Blocking 802 64 P2P 2 465792 0 8
lag256 Root Forwarding 1 64 P2P 5 465792 2 4
DC-8100-Agg1# sh span
Spanning tree status : Enabled Protocol: MSTP
MST0
Root ID Priority : 8192
MAC-Address: 02:02:00:00:01:00
Hello time(in seconds):2 Max Age(in seconds):20
Forward Delay(in seconds):15
Bridge ID Priority : 16384
MAC-Address: 02:01:00:00:05:00
Hello time(in seconds):2 Max Age(in seconds):20
Forward Delay(in seconds):15
Port Role State Cost Priority Type BPDU-Tx BPDU-Rx TCN-Tx TCN-Rx
------------ -------------- ---------- -------------- ---------- ---------------- ---------- ---------- ---------- ----------
......
lag5 Root Forwarding 800 64 P2P 9 558994 8 8
lag256 Designated Forwarding 1 64 P2P 606440 47893 20 12
Number of topology changes : 23
Last topology change occurred : 453774 seconds ago