MLAG vs LACP fast rate

Arista doesn't recommend LACP rate fast enabled with MLAG, which is not very reliable especially in scale environment.

For example:

mlagA/Primary  mlagB/Secondary
        \          /
         \        /
          \      /
          mlagLeaf

When either primary or secondary peer reloads, the mlagLeaf will see LAG flaps during this process. 

In 4.20.2.1F Release Note, on page 21

LACP PDU fast rate ("lacp rate fast" on an Arista switch) should not be configured on any port in an MLAG pair, or any port connected to an MLAG pair. (13950)

Reference:
bug13950/SR 101110/Old post