Rollups Explained: How Ethereum Layer 2s Work

Ethereum can process roughly 15 transactions per second. At peak demand, that ceiling creates congestion, and congestion creates fees. Transactions that cost pennies under normal conditions ran tens of dollars during the network activity spikes of 2021 and 2022.

Rollups fix this without changing Ethereum itself.

What Does a Rollup Do?

A rollup is a separate blockchain that processes transactions off Ethereum, compresses them into batches, and posts those batches to Ethereum for settlement. Ethereum provides the security, the rollup provides the throughput. Transaction fees drop to fractions of a cent and come with Ethereum's security guarantees.

Ethereum stores the data proving those transactions occurred and validates that the rollup's submitted state is consistent, without re-executing each transaction individually. Transactions are processed on the rollup, making them faster and cheaper while still benefiting from Ethereum's security.

How Does a Rollup Work?

A rollup processes transactions on its own execution layer, orders them via a sequencer, compresses thousands of transactions into batches, and posts those batches to Ethereum as blobs (since EIP-4844 in March 2024). A cryptographic state root is published to Ethereum at each batch and becomes the anchor for any fraud challenge.

Transaction ordering

Users submit transactions to the rollup's sequencer, the node responsible for ordering incoming transactions and building blocks. The sequencer collects transactions, orders them, and processes them on the rollup's execution layer.

Batch submission

The sequencer compresses batches of thousands of transactions and posts them to Ethereum. Since EIP-4844 (Ethereum's Dencun upgrade in March 2024), this data is posted as blobs, a dedicated data format that cut rollup posting costs by more than 90% compared to earlier calldata costs.

State commitment

The rollup publishes a state root to Ethereum at regular intervals. The state root is a cryptographic fingerprint of the rollup's complete current state. Ethereum stores this root, and it becomes the anchor for any fraud challenge. Anyone who believes a state root is wrong can submit one.

What Are the Two Types of Rollups?

Optimistic rollups assume transactions are valid and allow anyone to challenge an incorrect batch within a 7-day window. ZK rollups generate a cryptographic proof of validity with every batch, enabling near-instant finality. Most deployed L2 activity runs on optimistic rollups because of their full EVM compatibility with existing Ethereum applications.

Optimistic rollups

Optimistic rollups assume submitted transactions are valid by default. They do not submit a proof of correctness with each batch. Instead, they publish the state root and open a challenge window, typically 7 days, during which anyone can submit a fraud proof challenging an incorrect state.

If a challenger identifies an invalid state root and proves it on Ethereum, the incorrect batch is rejected and the operator who submitted it is penalized. If no valid challenge is submitted within the window, the state is finalized.

Optimistic rollups have full EVM equivalence. Smart contracts written for Ethereum run without modification. Any application that works on Ethereum works on an optimistic rollup with no code changes. This is why they dominate deployed activity: the migration path for existing Ethereum applications is immediate.

ZK rollups

ZK rollups generate a cryptographic validity proof with every batch. This proof mathematically guarantees that every transaction in the batch is correct before it is submitted to Ethereum. Ethereum verifies the proof, not the transaction data itself.

The cryptographic proof enables near-instant finality. There is no challenge window because validity is proven, not assumed. Assets can be withdrawn without waiting 7 days.

The tradeoff historically has been EVM compatibility. Generating ZK proofs requires mapping Ethereum's execution environment to a proof-friendly system, which is hard, and full EVM equivalence isn't finished yet. ZK rollup teams (zkSync, Starknet, Polygon zkEVM) have closed most of the gap, though some EVM edge cases remain.

What Is Data Availability, and Why Does It Matter?

Data availability determines whether Ethereum can verify and reconstruct a rollup's state if needed. Standard rollups post transaction data directly to Ethereum via blobs. That gives them full Ethereum-level security. Some rollups use external DA networks to reduce costs, which changes the security model and trust assumptions.

For a rollup to inherit Ethereum's security, Ethereum must have access to the transaction data it needs to reconstruct the rollup's state and verify any fraud proof. Where that data is stored is the data availability question, and the answer determines the rollup's security model.

Onchain data availability (standard)

Transaction data is posted directly to Ethereum. This gives the rollup full Ethereum-level security: even if the rollup's sequencer disappears entirely, the complete state history is recoverable from Ethereum.

EIP-4844 cut posting costs by more than 90% by introducing blobs, a dedicated data format for rollup data that expires after roughly 18 days. Users do not need historical data for normal transactions; the blob expiry does not compromise security for live chains.

Alternative data availability

Some rollups reduce posting costs by storing data off-chain, using networks like Celestia, EigenDA, or Avail rather than Ethereum itself. This lowers costs but changes the security model: the rollup's security now depends on the availability of the external network, not Ethereum directly.

For most production deployments, onchain data availability to Ethereum is the default. The cost savings rarely justify the added complexity at standard transaction volumes.

What Security Guarantees Does a Rollup Provide?

A rollup that posts data to Ethereum and maintains a working challenge mechanism inherits Ethereum's security for two properties: liveness (users can always withdraw funds even if the sequencer goes offline) and finality (once a state root is accepted on Ethereum, it cannot be reversed without reversing Ethereum itself).

Liveness means that even if the rollup's sequencer stops working, users can withdraw their assets directly from Ethereum using the rollup's bridge contracts. The sequencer going down does not strand funds.

Finality means that once a state root is finalized on Ethereum, either through the challenge window expiring for optimistic rollups or through a validity proof being accepted for ZK rollups, it cannot be reversed except by reversing Ethereum itself.

What Is Sequencer Risk?

Most rollups today run a single sequencer controlled by the chain's team. This sequencer determines transaction ordering and inclusion. If the sequencer goes offline, the chain can stop producing blocks, though user funds remain withdrawable via Ethereum bridge contracts. The sequencer is the primary trust assumption in any rollup.

Rollups introduce one trust assumption that Ethereum mainnet avoids: sequencer centralization. That sequencer controls transaction ordering, which means it controls:

• Which transactions get included and in what order
• Whether certain transactions are censored or delayed
• MEV extraction from transaction ordering

How Are Rollups Being Used in Production Today?

Rollups handle the majority of global Ethereum activity. OP Stack chains processed 6 billion transactions in 2025, 29x year-over-year growth, and accounted for more than 70% of all L2 transaction volume. Over $40 billion in assets is secured by rollup infrastructure across all L2 networks.

Frequently Asked Questions

Is a Rollup the Same as a Layer 2?

Layer 2 is the broader category: any network built on top of Ethereum that offloads execution. Rollups are the most common and most widely deployed type of Layer 2. Other Layer 2 designs exist (state channels, Plasma) but rollups now dominate because they support general-purpose applications and offer stronger security guarantees.

Do Rollups Have Their Own Tokens?

Not necessarily. OP Stack chains use ETH for gas by default. Some rollups issue their own governance or utility tokens, but this is a product decision, not a protocol requirement. ETH is the gas token on OP Mainnet and most OP Stack deployments.

Are Rollup Transactions as Secure as Ethereum Mainnet Transactions?

For optimistic rollups, a transaction on the rollup achieves equivalent security to Ethereum finality once the 7-day challenge window closes without a successful challenge. In practice, users and applications treat transactions as final much earlier because the economic cost of mounting a successful fraud attack is prohibitive. ZK rollup transactions achieve Ethereum-level finality immediately upon proof acceptance.

What Is the 7-Day Withdrawal Period For?

Optimistic rollups post state roots to Ethereum without an upfront proof. The 7-day window gives anyone who spots an incorrect state root time to submit a fraud proof before the state is finalized. Fast bridges, provided by third parties like Across and Stargate, allow immediate withdrawals by front-running the settlement and absorbing the 7-day wait themselves, for a small fee.

What Is the Relationship Between a Rollup and a Blockchain?

A rollup is a blockchain in the operational sense: it has blocks, a transaction history, accounts, and smart contract state. The distinction is where finality is anchored. An independent blockchain (like Ethereum) finalizes state internally. A rollup finalizes state by posting to Ethereum. The rollup's canonical history lives on Ethereum; the rollup's execution layer handles the transaction processing.

Glossary

Rollup - A blockchain that processes transactions off Ethereum and posts compressed transaction data to Ethereum for settlement and security.

Sequencer - The node responsible for collecting, ordering, and processing transactions on a rollup's execution layer.

State root - A cryptographic fingerprint of the rollup's complete current state. Published to Ethereum at regular intervals as the anchor for fraud challenges.

Fraud proof - A cryptographic challenge submitted to Ethereum proving that an optimistic rollup's submitted state root is incorrect.

Validity proof - A cryptographic proof generated by a ZK rollup that mathematically guarantees every transaction in a batch is correct. Ethereum verifies the proof directly.

Blob - A dedicated data format for rollup transaction data, introduced in EIP-4844 (March 2024). Reduces rollup posting costs by 90%+ vs. calldata. Expires after ~18 days.

Data availability (DA) - Whether Ethereum or another network has access to the transaction data needed to verify or reconstruct a rollup's state.

Finality - The point at which a transaction is irreversible. For optimistic rollups: after the 7-day challenge window. For ZK rollups: immediately upon proof acceptance.

EVM equivalence - The property of a rollup's execution environment matching Ethereum's exactly, so all Ethereum smart contracts run without modification.