I recently came across the fact that my favorite IGP, the Intermediate System To Intermediate System (IS-IS) Routing Protocol has strict requirements regarding the minimum MTU of a link. In fact, for your adjacency to come up, the IS-IS link's maximum transmission unit (MTU) must be at least of 1492 bytes.
There's actually a few ways to avoid bridging loops in a VPLS network. Bridging in a VPLS environment is not really different from a standard Ethernet network, a spanning-tree protocol like the original IEEE 802.1D or any of its variants like RSTP or MSTP can be enabled to block the redundant link(s). Ethernet Ring Protection (ERP) could also be enabled on platforms supporting it (e,g. Juniper MX series), but certainly the most common and effective way is to carefully provision the VPLS VPN instances using BGP and to respect a few basic rules.