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This is regarding LACP System Priority.

From my research I could find the following info pertaining to system priority.

LACP combines the System Priority value with the MAC address of the switch to create a System ID. The switch with the lower System ID becomes the master switch, and the other switch must fall into line with whatever decision the master makes reg ports to be used in LACP.

My query here is does this negotiation is indicated via a field in packet saying one switch is master and other is standby.

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    Have you looked at the 802.1ax standard? All your LACP questions will be answered there.
    – Ron Trunk
    Sep 7, 2020 at 15:45
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin ♦
    Dec 17, 2020 at 21:21

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There's no master switch with LACP. The System ID is used to match ports belonging to the same trunk group.

You should look up the original IEEE 802.1AX standard. It's free after registration on https://ieeexplore.ieee.org/browse/standards/get-program/page/series?id=68

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For nearly all modern purposes, it really doesn't matter. There is no "master/leader" role for LACP from my experience.

The switch with the lower LACP system priority gets to choose which link are active when there's some sort of limitation on which links can be active in a bundle. For example, some systems will let you have 1G and 10G links in a bundle, but only let 1G or 10G active.

It's not common anymore, but I vaguely remember certain switches/routers that would only let you configure LACP if it was on the same ASIC or something like that. Regardless, one side of the LACP link should have the same system priority.

If you're running any sort of MLAG setup, the two switches acting as one need to have the same system priority or the remote system won't see them as the same switch. This goes for vendor-proprietary MLAG and newer standardized EVPN-multihoming solutions.

It's worth noting that LACP port priority serves a different purpose.

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