If a RaaS provider had many instantiated rollups, then to have interoperability across the rollups, one would normally build an enshrined bridge on the DA layer, one per rollup. An alternative would be to have one bridge between every two rollups that use the same DA layer.
With AltLayer, we adopt a third design that leverages the Beacon chain to serve as a bridging hub to coordinate across all rollups. In this design, every rollup would enshrine a bridge into the Beacon layer that then acts as a shared intermediate “settlement layer” on top of the underlying DA layer. By enshrining a bridge to this shared interlayer, rollups as a result get a direct trust-minimized bridging with it. Additionally, this also yields indirect trust-minimized bridging to other rollups that also enshrine bridges to the Beacon Layer. In essence, as a common settlement layer, the Beacon Layer acts as a bridging hub that facilitates asset transfers between rollups but also general message passing. It could also help detect any invalid state transition across rollups and even provide dispute resolution for verifying fraud proofs.