A layer 2 is a separate blockchain that runs on top of a base chain like Ethereum to make transactions faster and cheaper, while still relying on the base chain for security. The dominant kind of layer 2 is the rollup: it processes transactions off to the side, then posts a compressed record of them back to Ethereum. You get low fees and quick confirmations, but Ethereum remains the ultimate source of truth.
The problem layer 2s solve
Ethereum's main chain (layer 1) is secure and decentralised, but its block space is limited, so when demand is high, fees rise and transactions slow down. You could make the base chain itself bigger and faster, but that tends to push out everyday participants who run the network, weakening decentralisation.
Layer 2s take a different route. Instead of making layer 1 handle more, they handle transactions elsewhere and lean on layer 1 only for final security. This is often called scaling Ethereum "off-chain" while keeping its guarantees.
How a rollup works
A rollup runs its own execution environment where transactions happen quickly and cheaply. Periodically it rolls up many of those transactions into a batch and posts that batch to Ethereum. Two ingredients make this trustworthy:
- Data posted to layer 1. The rollup publishes enough transaction data to Ethereum that anyone could reconstruct its state independently. This is the data availability guarantee — the rollup cannot hide what it did.
- A proof that the batch is valid. The rollup must convince Ethereum that the new state really follows from the old one under the rules. How it proves this is what splits rollups into two families.
A component called the sequencer orders incoming transactions and produces batches. Most rollups today run a single sequencer, which is efficient but a point of centralisation the ecosystem is working to decentralise.
Optimistic vs zk rollups
| Optimistic rollup | Zero-knowledge (zk) rollup | |
|---|---|---|
| Core assumption | Batches are valid unless challenged | Every batch comes with a maths proof of validity |
| How fraud is caught | Anyone can submit a fraud proof during a challenge window | Invalid batches can't be proven, so they're rejected outright |
| Withdrawal delay to L1 | Long (about 7 days) to allow challenges | Short, once the proof is verified |
| Examples | Arbitrum, Optimism, Base | zkSync, Starknet, Scroll, Linea |
Optimistic rollups
An optimistic rollup assumes each posted batch is honest and lets it through by default. To keep it honest, there is a challenge window (typically about seven days) during which anyone watching can submit a fraud proof — evidence that a batch broke the rules. If they do, the bad batch is reverted and the cheater is penalised.
The upside is simplicity and full compatibility with Ethereum's tooling. The downside is the delay: because a challenge could still arrive, withdrawing funds straight back to layer 1 takes about a week (third-party "fast bridges" offer quicker exits for a fee).
Zero-knowledge rollups
A zk rollup takes the opposite stance: it attaches a validity proof (often a zk-SNARK or zk-STARK) to every batch. This is a compact piece of cryptography that mathematically proves the new state is correct, which Ethereum can check cheaply. Nothing is accepted without a valid proof, so there is no need to wait for challenges, and withdrawals settle much faster.
The trade-offs are technical: generating proofs is computationally heavy, and building a zk system that matches Ethereum's behaviour exactly has been hard. That gap has been closing quickly.
What made rollups cheap: blobs
Posting data to Ethereum used to be a rollup's biggest cost. A 2024 upgrade, EIP-4844 (often called "proto-danksharding"), introduced blobs — a dedicated, much cheaper lane for rollups to post their data, which is kept available for a short time and then discarded. This cut layer-2 fees dramatically and is a major reason transactions on rollups now cost cents rather than dollars.
Using a layer 2, and what to watch
In practice you bridge assets from Ethereum to the rollup, then transact there with the same wallet and apps at a fraction of the cost. A few things to keep in mind:
- Bridging risk. Moving funds between chains relies on bridge contracts, which have historically been a target for hacks. Prefer a rollup's official bridge, and be patient with optimistic-rollup withdrawals.
- Sequencer trust. A single sequencer could censor or reorder transactions in the short term; it cannot steal funds, because layer 1 still enforces validity and data availability.
- Maturity varies. Rollups differ in how decentralised and battle-tested they are. Independent trackers rate how far each has progressed toward removing training wheels.
Because this area moves fast, treat specifics — fee levels, withdrawal times, how decentralised a given rollup is — as a snapshot and check current details before moving significant value.
Key takeaways
- A layer 2 runs on top of a base chain to cut fees and speed up transactions while inheriting the base chain's security.
- A rollup executes transactions off-chain, then posts compressed data and a validity guarantee back to Ethereum.
- Optimistic rollups assume validity and rely on fraud proofs within a ~7-day window; withdrawals to L1 are slow.
- Zk rollups attach a cryptographic validity proof to every batch, so withdrawals settle fast but the tech is more complex.
- The EIP-4844 "blobs" upgrade gave rollups a cheap data lane, slashing layer-2 fees.
- Watch bridge risk, single-sequencer trust, and each rollup's maturity before moving large amounts.
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