Symptom
For a GLC-T transceiver, a Cisco ACI switch signals a peer device to turn on flow control even though LLFC is disabled since the 14.1(1i) release. This behavior can cause a vPC down between ACI switch and peer IOS device like Cat6k with flow control desirable/auto. When one ACI switch running version before 15.2(7f), the other has 15.2(7f) or beyond, Cat6k would have one port with flow control on but the other off, hence fail to aggregate the port-channel.
Conditions
This issue occurs with a Cisco ACI-mode switch with GLC-T. The ACI-mode switch forms a vPC with a peer Cat6K whose flow control is auto/desirable.
Workaround
Disable flow control on the peer IOS device before below upgrading path:
1) upgrading ACI earlier than 14.1(1i) to any version earlier than 15.2(7f).
2) upgrading ACI earlier than 15.2(7f) to any version later than 15.2(7f).
