Bridging & Interoperability (CDK, LxLy Bridge, Unified Bridge & AggLayer)

Bridges are an essential part of L2s, enabling users to deposit and withdraw assets between your chain and other networks.

CDK-built chains include a built-in bridge service and a customizable UI by default. Users can choose a standalone LxLy bridge or opt into the AggLayer, leveraging the unified bridge for seamless L2-to-L2 cross-chain interoperability.

We Have Identify Three (3) types of bridges 

1. Native Bridge: Built specifically for an L2, allowing deposits and withdrawals from L1.

2. Third Party Bridge: External solutions that facilitates cross-chain transfers between multiple networks 

3. Liquidity Bridge: Use liquidity pools instead of waiting for finality on the source chain, making transfers faster.

CDK
Polygon Chain Development Kit (CDK) is a modular, open-source software toolkit for blockchain developers that supports the installation and configuration of ZK-powered L2s. There are two primary configurations for chains built with CDK: rollup and validium.

LxLy Bridge 

The LxLy bridge contracts facilitate asset deposits and withdrawals between L2 and L1.

Chains that prefer to manage their own bridge infrastructure can deploy a dedicated instance of the LxLy bridge, enabling users to transfer both native assets and ERC20 tokens between L1 and L2.

Deploying an individual instance of the LxLy means interoperability with other L2 chains via the AggLayer is not possible. To enable cross-chain interoperability (i.e. L2-to-L2 cross-chain transactions), chains can opt-in to the AggLayer and use the unified bridge 

Using the native bridge i.e the LxLy bridge enables chain  to customize how the bridge is managed and operated LxLy bridge allow chain operators to have control of the bridge funds, since the bridge contract are managed by the chain operator

Unified Bridge 

The unified bridge (prev. LxLy bridge) is the common access point for chains connected to the Agglayer. Along with the pessimistic proof, it is one of two core components of the Agglayer.

The unified bridge is a single shared instance of the LxLy bridge available to use for all CDK chains that opt in to the AggLyer. It is a shared smart contract deployed on Ethereum, rsponsible for interoperability between chains in the form of cross-chain transaction and L2 to L2 transfers.

Chains that integrate with the unified bridge can benefit from the network effect of the AggLayer, as their chain can therefore access the users and liquidity of other chains that are also part of the AggLayer. 

This option is suited to chains that want a standard bridging experience and do not require customization of the bridge’s operation. The shared bridge is also not directly managed by the chain operator, instead, it shares the governance outlined in the admin upgradeability section.

AggLayer

The Agglayer is an in-development interoperability protocol that allows for trustless, cross-chain token transfers and message-passing, as well as more complex operations. The safety of the Agglayer is provided by ZK proofs.

The latest version of the Agglayer (v0.2) connects chains built with Polygon CDK, a developer toolkit for designing ZK-powered Layer 2s.