Dumlu,
You are right that the peer-switch feature has changed the way that the two vPC peers send BPDUs out of their local vPC member ports. With the peer-switch feature, both vPC peers (operationally primary and operationally secondary) send BPDUs out of their vPC member ports toward Layer 2 access switches connected to the vPC system with vPCs. The BPDUs that they send contain the same bridge ID, which means that they are using the same bridge priority and the same system MAC. This system MAC should be automatically generated and contain the vPC domain ID in its least significant bits. If you don't want the vPC system to use this automatically generated and shared between the two vPC peers MAC address, you can use the system-mac command on both vPC peers to manually assign a shared MAC address that would be used in BPDUs sent out of local vPC member ports by both vPC peers.
Before the peer-switch feature was introduced, only the operationally primary vPC peer would send BPDUs out of its local vPC member ports. These BPDUs contained the bridge ID of the STP root configured on one of the vPC peers. However, if the STP root is currently operationally secondary vPC peer, then the operationally primary vPC peer sends BPDUs out of its local vPC member ports, but it uses the other vPC peer's (the STP root but operationally secondary vPC Peer) system MAC in the BPDUs. The operationally secondary vPC peer (even if it is configured as the STP root) would forward BPDUs received on its vPC member ports from Layer 2 switches onto the operationally primary vPC peer across the vPC peer link.
The same system-mac command has a similar effect in LACP, where LAGID consists of the system ID and the channel-group number. The system ID consists of the system priority and the system MAC. That system MAC is created automatically on both vPC peers with the least significant bits of the last octet populated with the vPC domain ID. Just like with the system MAC address used in STP BPDUs, you can override this automatically generated MAC address, using the same system-mac command on both vPC peers, except that if you do this, LACP will always use the system MAC that you manually configured. Again, STP will only use this system MAC (or automatically generated system MAC for that matter) if you enable the peer-switch feature.
The behavior you are describing seems to be consistent with the pre-peer-switch feature behavior. With that *legacy* behavior, if the operationally primary vPC peer was the same switch as the STP root, it would use its own system MAC in the BPDUs sent out of its local vPC member ports. This MAC address would not be shared between the two vPC peers. When the operationally primary vPC peer, which is also the STP root, would go down, the operationally secondary vPC peer would start sending BPDUs containing its own MAC address out of its local vPC member ports. Therefore, a Layer 2 switch connected to the vPC system with a vPC would see a new root (with a different MAC address). In this case, the system-mac command has no effect on which MAC address is used in BPDUs sent by only one vPC peer at a time. The behavior gets even more strange in the Active/Active (split-brain) scenario, where both vPC peers start forwarding BPDUs out of their local vPC member ports, using their own MAC addresses. The downstream Layer 2 switches would then see two different STP roots on of the same PortChannel interface because the BPDUs from both vPC peers in a split-brain condition will arrive on different links that are members of the same PortChannel on the Layer 2 switch. Therefore, I would much rather use the peer-switch feature now that it has become available.
Now, I have a question for you, How do you look up the MAC address used in LACP? I don't seem to be able to find a command for this.
Thanks!
