Hi Twigs,
I think it might be better to make a new post instead of replying to this one. In general, the port speed difference will not cause a license error.
I would suggest you run "system license show" on all three clusters and verify that there are snapmirror licenses installed for each node on each cluster - the system will let you install licenses for nodes not in the cluster, so if there is any mixup, the licenses may show but not be in effect.
