Rollup to Rollup Bridges: Connecting Layer 2s

Rollups are some of the most popular forms of L2 scaling solutions. Optimistic rollups currently have $5.06 billion in value locked across five different solutions and ZK rollups have a TVL of $1.47 billion, both of which have been growing with time. As Ethereum fees continue to rise with demand, the interest in rollups is growing as they can offer fees significantly lower than the mainnet. With all of the rollup solutions being developed, it is not easy to choose only one. For instance, a user could bridge some ETH to Optimism but then want to use SushiSwap to exchange that ETH to FTM without mainnet fees. However, since SushiSwap is only integrated with Arbitrum, the user first must bridge their ETH back to the Ethereum mainnet from Optimism and send it to Aribtrum. This process is full of friction and defeats the entire purpose of using a rollup at all since the user must transfer through an L1 in order to get to the other rollup. It is also incredibly time-consuming because of the 7-day challenge period the user incurs before they can withdraw their funds from Optimism. As every rollup option continues to develop its own ecosystem, this friction will continue to grow. This is where rollup to rollup bridges come into play. These bridges alleviate the gaps between rollup solutions and allow users to transfer between them without needing to use the expensive mainnet.

The concept of bridging assets from one location to another is not foreign. Most rollup solutions have a native token bridge that takes tokens from an L1 and "moves" them to an L2 utilizing a mint and burn method in which users lock funds in a contract on the source layer and then mint a canonical version of the token being bridged on the destination rollup. To transfer tokens back to the mainnet, the canonical token on the rollup is burned and the funds locked in the L1 contract are released. Similarly, there are cross-chain bridges that work in essentially the same way, moving assets between L1s. Thus, it was a natural extension to consider bridging assets from scaling solution to scaling solution. There are two main projects that are live for enacting these cross-layer transfers, Celer Networks' cBridge and Hop Protocol. We will be looking at how each of them works and their adoption so far.