[chillyjim]

This has been said but not clearly in my book.

Short answer the firewall interfaces need to be real/not-vlan interfaces. The switch side of the connection can cross switches via a trunk port.

Longer answer -- Make sure the trunk has enough bandwidth to handle the sync traffic (I've seen over 800 Mbps on a sync connection). You also have to worry about latency. If it's all "local" fiber you should be OK, but leased fiber from a carrier is as likely as not, not to work.