In XOS 15.5 I had a problem in the past, that mac learning were not correct working, when mlag peers run different 15.5 versions.
I personally would recommend to upgrade the g1 to last 15.7 or 16 and the g2 to XOS 21.
I don't like XOS Version 15.6 and prior. to much trouble. I know, it's recommended for a lot of systems. But I'm running at least XOS 15.7 on all my customers and have no problems.
Last year in july I set up a new core with 2 X670G2 as MLAG Peer with Vrrp on XOS 15.7 at a customer. Until March this year no problems. In March I upgrated to XOS21 with nearly no traffic interrupt.
In the near future I will configure vrrp fabric routing (activ/active).
Peter thanks for your input ya i'm wondering if i shouldn't just jump to XOS21... i was worried since it was such a big version jump it would be buggy... but honestly 15.5 seems to be very buggy even though it was the recommended release at the time.
the path architecture between branches seems sooo crazy i mean you would think that any patch with a publish date later than the original patch that had a fix would include the fix from said problem. I think one of our problems with the 15.5 releases is we're hitting a xos bug that causes Unable to Add IPmc sender entry s,G,v=x.x.x.x,220.127.116.11,2022 IPMC 8 flags 4 unit 0, Entry exists which i believe, which might be whats affecting ospf... and then we have random reboots on both of our XOS boxes that are OSPF/VRRP routers.
The recommended software list is sorta funny considering it doesn't recommend latest branches, but on top of that it doesn't even recommend latest patch version of those releases.