ACI fabric does not run spanning-tree. By default, an EPG in ACI will just flood BPDUs (even if Hardware proxy mode is chosen for the BD).
The downstream Spanning-tree switches are supposed to run Rapid-PVST+
Here we assume
- BD name - Lab1
- EPG in that BD with 4 paths all with encapsulation vlan-1110
-- on leaf1 port-channel to Nexus7k-50 (Node101/Policy_to_sw50) - port 101/1/11,13 to port-channel1 on nexus7k-50
-- on leaf1 port-channel to Nexus7k-65 (Node101/Policy_to_sw65) - port 101/1/17-18 to port-channel1 on nexus7k-65
-- on leaf3 port-channel to Nexus7k-50 (Node103/Policy_to_sw50) - port 103/1/11,13 to port-channel2 on nexus7k-50
-- on leaf3 port-channel to Nexus7k-65 (Node103/Policy_to_sw65) - port 103/1/17-18 to port-channel2 on nexus7k-65
This is the topology we are using :
Below shows the Bridge domain configuration:
Below shows the path part of the EPG:
From the spanning-tree point of view, the ACI fabric acts as a wire and the logical spanning-tree topology can be seen as:
The following shows the spanning-tree status in vlan 1110 for both switches attached to ACI:
n7k-50 is not the root of the spanning-tree and it has a root port on port-channel 1 pointing to the ACI fabric.
Po2 is alternate blocking as expected
output from N7k-50:
N7K-50# sh spanning-tree vlan 1110
VLAN1110
Spanning tree enabled protocol rstp
Root ID Priority 5206
Address 001b.54c2.2641
Cost 1
Port 4096 (port-channel1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 33878 (priority 32768 sys-id-ext 1110)
Address 0026.980a.df41
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1 Root FWD 1 128.4096 P2p
Po2 Altn BLK 1 128.4097 P2p
on N7k-65 we have the following.
Note :
- N7k-2-65 is the Root of the spanning in vlan 1110
- Po1 is Designated port
- Po2 is Backup blocking (as blocked by a bpdu from itself). this proofs ACI floods BPDU.
N7K-2-65# sh spanning-tree vlan 1110
VLAN1110
Spanning tree enabled protocol rstp
Root ID Priority 5206
Address 001b.54c2.2641
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 5206 (priority 4096 sys-id-ext 1110)
Address 001b.54c2.2641
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1 Desg FWD 1 128.4096 P2p
Po2 Back BLK 1 128.4097 P2p
