What Are Layer-2 Rollups? Ethereum Scaling Explained

In short: Ethereum's main chain (layer 1, or L1) is deliberately kept lean so that many people can run it. Layer-2 networks (L2s) take on the heavy lifting off-chain, and "rollups" are the approach Ethereum's roadmap currently centres on.

Why L2s exist

According to ethereum.org, Ethereum cannot easily scale its own main chain, because making it faster would require larger nodes and more specialised hardware, shrinking the number of people who can run one. Instead, Ethereum focuses on being a secure settlement layer, and other networks build on top of it. The site notes that the Fusaka upgrade (December 2025) introduced PeerDAS, a more efficient way for L2s to post and retrieve data on Ethereum.

What a rollup does

A rollup executes transactions outside Ethereum, bundles many of them into a batch, and posts the resulting data to Ethereum. Spreading the fixed costs of posting across many users is what lowers fees. Rollups publish data either as "calldata" or in "blobs", a cheaper format introduced by EIP-4844; ethereum.org says the network only has to serve blob data for a minimum of roughly 18 days. There are two main flavours.

Optimistic rollups

Optimistic rollups assume transactions are valid unless someone proves otherwise. After a batch is posted, there is a challenge period during which anyone can submit a "fraud proof". The documentation says withdrawing to Ethereum takes roughly seven days because of this window, though liquidity providers can speed it up for a fee.

ZK-rollups

ZK-rollups attach a cryptographic validity proof to each batch, which Ethereum verifies before accepting the new state. That means no fraud-proof waiting period for withdrawals. The trade-off is complexity: ethereum.org lists high proof-generation costs and difficulty building EVM-compatible versions among the drawbacks.

Not every L2 is equal

Ethereum.org stresses that there is no "official" Ethereum L2, and that safety depends on the underlying technology, smart-contract security and maturity. It suggests users do their own research and start with small transactions. The documentation also flags concrete risks. In both rollup types a central "sequencer" can influence the ordering of transactions. In optimistic designs, security relies on at least one honest party checking the chain.

Quick comparison

Optimistic rollups show correctness through fraud proofs, if someone challenges a batch; withdrawals to Ethereum take roughly a week by default, which is their main trade-off. ZK-rollups show correctness with a validity proof on every batch and have no challenge delay, but face proof complexity and cost.

Why it matters now

Ethereum's developers are also working on the base layer. As of October 2026, the next upgrade, Glamsterdam, is aimed at increasing throughput on L1 itself and is heading to a test network; see Ethereum's Glamsterdam Upgrade Hits Sepolia on Oct. 6. Rollups and base-layer upgrades are meant to work together. To see why Ethereum's security model matters here, read our guide to proof of stake vs proof of work.

This article is educational and is not financial advice.

Sources

• ethereum.org, Intro to Ethereum Layer 2
• ethereum.org, Optimistic Rollups
• ethereum.org, Zero-knowledge rollups