Summary

Adding a second member interface to an existing bond re-assigns the bond's MAC address to that new member. The bond changes its identity on the wire, breaking ARP/ND and LACP for peers that already learned it.

Steps to reproduce

set interfaces bonding bond0 mode 802.3ad
set interfaces bonding bond0 member interface eth1
commit

run show interfaces bonding bond0    # note link/ether of bond0
run show interfaces bonding bond0 slaves
Interface        RX: bytes  packets     TX: bytes  packets
bond0            0          0           0          0
    eth1         0          0           0          0

set interfaces bonding bond0 member interface eth2
commit

run show interfaces bonding bond0    # link/ether is now eth2's MAC
run show interfaces bonding bond0 slaves
Interface        RX: bytes  packets     TX: bytes  packets
bond0            0          0           0          0
    eth2         0          0           0          0

Actual

bond0 and eth2 share aa:c1:ab:c3:62:48; the address the bond had from eth1 is gone.

If both bond members are added simultanously, no issue exists.

Expected

bond0 keeps the MAC it adopted at creation time. Appending a member must not change it, nor remove it from the slave list.

T7571 made the bond adopt its first member's MAC, since newer kernels assign a synthetic one. In BondIf.update() the driving flag is initialised True unconditionally, and an already enslaved member hits continue without clearing it, so the adoption re-runs on every append.