I have recently seen a webinar from ipSpace
about Cumulus Linux, where it was said that Cumulus does not support ISSU (in service software upgrade). Some vendors call the possibility to upgrade one switch of an MLAG pair at a time MLAG ISSU. Those vendors typically check the MLAG protocol version of the peer and do not establish an MLAG peering if the versions do not match and are not known to be ISSU compatible. This situation is usually only supported during software upgrade, i.e. the other peer is supposed to be upgraded, or the upgrade rolled back, in a reasonable time window.
How does Cumulus handle the case of two MLAG peers using different Cumulus Linux versions?
(Edited for spelling)