Layer 2 networks and bridges: what changes?

Ethereum can process only a limited number of transactions per block, and when demand is high, fees rise. Layer 2 networks move most of the work off the main chain and post the results back to Ethereum. Bridges move assets between networks. Both can make transactions cheaper or faster, and both add assumptions worth understanding before you rely on them.

Key takeaways

• A layer 2 executes transactions on its own chain and relies on Ethereum for settlement and data. Fees are lower because many users share the cost of each Ethereum transaction.
• Optimistic rollups assume transactions are valid unless someone challenges them. Zero-knowledge rollups prove validity with cryptography.
• “Layer 2” is a broad label. Sequencers, upgrade controls and data availability differ from network to network.
• A bridge is a separate system with its own risks, and the same token symbol on two networks can be two different assets.

Why another layer?

Every Ethereum node re-executes every transaction. That keeps the network verifiable but limits throughput. A rollup runs transactions on its own chain, compresses them and publishes the data to Ethereum in batches. One Ethereum transaction can then carry the activity of thousands of users, so each pays a small share. Since the Dencun upgrade in March 2024, rollups can post this data in blobs, a cheaper, temporary form of storage designed for them.

Two main rollup designs

Optimistic rollups publish results and assume they are correct. Anyone watching can submit a fraud proof during a challenge window, so withdrawals through the official bridge wait for that window to close, which on major networks takes about a week. Zero-knowledge rollups attach a validity proof to each batch, and Ethereum verifies it directly. Once a proof is accepted there is no challenge period, although generating proofs and batching still take time.

Other designs keep transaction data off Ethereum altogether to cut costs further. They inherit less of Ethereum’s security, because users depend on someone else to keep that data available.

A shared label, different designs

Many layer 2 networks still rely on a single sequencer, run by the team, to order transactions, and many have upgrade keys or security councils that can change the contracts. These are often described as temporary arrangements on the way to fuller decentralization, but they matter today. Independent trackers such as L2BEAT publish risk assessments that show, network by network, who can censor, pause or upgrade the system.

Moving between networks

A bridge locks or burns an asset on one network and releases or mints a representation of it on another. A network’s canonical bridge is built into its rollup contracts and shares its security model. Third-party bridges use their own validators, liquidity pools or messaging systems, and they are often faster. Bridges have suffered some of the largest exploits in crypto, because a single contract can hold a large amount of locked value.

Bridging also creates lookalike assets. A token bridged by one route can be a different contract from the same token issued natively on the destination network, even though both show the same symbol. Some applications accept only one version. Check the contract address on the destination before you bridge or trade.

Worked example: one transfer, three questions

Suppose you want to move USDC from Ethereum to a layer 2. First, which network exactly? Confirm the chain name and chain ID in your wallet. Second, which route? The network’s canonical bridge, the issuer’s own transfer service and third-party bridges differ in timing, fees and trust. Third, which asset arrives? Native USDC on the destination and a bridged version can have different contract addresses. Answer all three before you sign.

What to check

• Use the network’s official documentation to find its canonical bridge and supported wallets.
• Read the withdrawal process before you deposit. A week-long exit is normal on some networks; it is a problem only if you did not expect it.
• Look up the network’s risk profile on an independent tracker.
• Confirm the destination token contract with the issuer or in the network’s documentation.
• Send a small amount first, and make sure you can pay gas on the destination network once the funds arrive.