MST TCN BPDUs interaction

[Antonio Macia]

Hi,

Per the documentation, every time ACI receives a TCN BPDU, it flushes the endpoint database for the EPG associated with the VLAN where the TCN came from. This makes sense when the ACI fabric is dual connected with the legacy network through two different leafs (without VPC), to avoid traffic blackholing when one of the legacy paths fails and the traffic must go through the alternative leaf.

However, when there is a single VPC between two leafs and two legacy switches (also in VPC), there is no alternate paths and TCN BPDUs will flush the endpoints for nothing, causing traffic flapping in ACI. Should I configure the VPC port at the legacy switches side as "spanning-tree portfast trunk" to prevent BPDUs from reaching ACI to avoid EP flushing? Would this cause any risk? My set-up uses MST, but I assume this is not relevant in this case.

Thanks.

[Remi-Astruc · Accepted solution]

Hi @Antonio Macia ,

Yes, if you are sure there won't be any other Legacy connections, you can definitely do that.

No specific risk, but like on traditional switches with no-loop design, some prefer to keep spanning tree to prevent an unexpected loop caused by miscabling/misconfiguration.

[Antonio Macia]

Thank you Remi.