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Spanning-tree - Rapid PVST+ switches interaction with ACI

Spanning-tree - Rapid PVST+ switches interaction with ACI

 

 

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

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