What is a Rollup?
A rollup is a scaling solution that processes transactions off a main blockchain in batches, then posts the results and supporting data back to it. Ethereum layer-2 networks such as Arbitrum, Optimism, Base and zkSync are rollups.
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A rollup is a scaling solution that executes transactions off a main blockchain, bundles them together, and posts the results plus the data needed to verify them back to the main chain. Because hundreds of transactions share the cost of a single submission, fees per transaction drop sharply. Most layer-2 networks, including Arbitrum, Optimism, Base, zkSync and Starknet, are rollups, and Ethereum’s scaling roadmap is built largely around them.
How does a rollup work?
- Users send transactions directly to the rollup.
- A component called the sequencer orders them and gives users a soft confirmation within seconds.
- Transactions are compressed into a batch, which is posted to Ethereum along with a summary of the new state.
- A smart contract on Ethereum secures the result, either through a challenge process or a cryptographic proof.
What makes a rollup a rollup is that the transaction data is published on the main chain. Even if the operator vanished, anyone could use that data to reconstruct balances, and users could withdraw their funds to mainnet.
Optimistic rollups vs. ZK rollups
- Optimistic rollups: results are assumed to be correct. During a challenge window, anyone can dispute a bad result with a fraud proof. Arbitrum, Optimism and Base use this model, and withdrawals to Ethereum through the official bridge typically take about seven days.
- ZK rollups: each batch comes with a validity proof, a zero-knowledge proof showing the computation was done correctly. Once Ethereum verifies the proof, the batch is accepted, so withdrawals are usually faster. zkSync Era, Starknet, Scroll and Linea belong to this group.
Why are rollup fees so low?
A rollup’s biggest cost is posting data to Ethereum. EIP-4844, shipped in the Dencun upgrade in March 2024, gave rollups a cheaper, temporary data space known as “blobs.” Fees on many layer-2 networks fell noticeably after the change. Our gas fee guide explains how fees are calculated more generally.
What are the risks of using a rollup?
Theory and current practice don’t always line up. On many rollups the sequencer is run by a single company, so if it goes down, transactions can be delayed or censored. Some teams can still upgrade the core contracts on short notice, and the maturity of fraud-proof or validity-proof systems varies from network to network. Independent trackers such as L2BEAT classify rollups into stages (Stage 0, 1 and 2) based on these criteria.
For everyday users, the most common problem is network confusion. Your wallet may show the same address on mainnet and on a rollup, but the balances are separate. Moving funds requires a bridge or an exchange that supports both networks. Fast-withdrawal services cut the waiting time but add fees and counterparty risk.
Frequently asked questions
Is a rollup the same as a layer 2?
Rollups are the most common kind of layer 2, but not every layer 2 is a rollup. Designs such as validiums keep transaction data off Ethereum and rely on different security assumptions.
Why does withdrawing from a rollup to Ethereum take so long?
With optimistic rollups you wait for the challenge window to close. With ZK rollups a proof has to be generated and verified on Ethereum, which is usually quicker.