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MLAG is an active-active technology and there is not concept of a primary and a secondary device. You can read more about it here: https://eos.arista.com/mlag-basic-configuration/. Did I misunderstand your questions by any chance?
Well what I mean is when configure MLAG on a switch pair and perform a ”show mlag detail” I see that one is primary and the second switch is secondary. See below. How are the switches choosing which one is secondary and which one is primary. Because from the perspective of the end host it sees the MAC address of the primary MLAG switch.
Arista1#sh mlag det
MLAG Detailed Status:
Arista2#sh mlag det
MLAG Detailed Status:
Victor – The primary MLAG peer is selected based on the lowest MAC address of the two peers, this address is selected and the most significant bit of the address is flipped making a link local multicast address, based on your output:
The system ID for the primary is: 00:0c:29:92:5c:be
The system ID for the secondary is: 00:0c:29:ce:f6:26
The system address is: 02:0c:29:92:5c:be (NOTE: this is the primary’s address with the MSB flipped)
The two systems share this system ID to communicate with neighbors (e.g. LACP, BPDUs) so the neighbors believe they are talking to the same switch. The switches use a sync of their state that is passed over the peer link to communicate any changes to the state of the protocols.
The end host will only see the system ID, not the MAC address of the switch, unless you are referring to routing and the ARP table in this case the ARP entry will point to the MAC address of the default gateway, not the MLAG system ID.
Thank you that was a good explanation.
I’m confusing with the system-id.
Why the system-id is: 02:0c:29:92:5c:be ?
As my knowledge the system-id is mac of primary device but i’m not sure this.
Could you help me explain it ?
As mentioned by Mark, the System ID is used by the MLAG Peers to communicate with neighbors such that they appear to be a single logical switch in the network (at Layer 2). That said, when the MLAG is formed for the first time, the lowest MAC address among the 2 peers is taken and the MSB bit is flipped. In this case, 00:0c:29:92:5c:be (lower MAC) becomes 02:0c:29:92:5c:be (since this is will become a unique MAC) that can be used as the System ID by the MLAG peers.
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