Dietrich,
initially it was necessary for this link to be FE.
With the most recent versions this link can also be a GE link.
Since this link is only being used for some kind of keepalive between the 2 CSS, I don't see the need to use GigE.
Also, unless your customer is doing firewall loadbalancing, you should really convince him to use vip/interface redundancy and not box-to-box redundancy.
You get faster failover time with vip/interface red., you can turn on statefull failover [not possible with box-to-box] and it is easier to troubleshoot.
Regards,
Gilles.
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