each MX needs to be able to see the other MX on each VLAN (the MX runs VRRP between each internal layer 3 interface).  You can use access interfaces to achieve this if you like, but trunk interfaces tend to be more efficient on ports when there are more than just a handful of VLANs.  There is also nothing to stop you using both trunk and access interfaces at the same time, as needs may require. ... View more