CRYPTO GUIDANCE INC.

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What Is a Rollup?

Published 2026-08-15. Last updated 2026-08-15. Written by John Alewine, Crypto Guidance Inc.

A rollup is a Layer 2 that executes transactions off the main Ethereum chain, then posts enough data and proof back to Ethereum that anyone can check the result. That is the difference between a rollup and a sidechain. The sidechain asks you to trust its validators. A honest rollup asks Ethereum to hold the receipts.

Who This Is For

Anyone paying "Ethereum fees" on Base, Arbitrum, Optimism, zkSync, or a similar chain and wondering what they actually bought. It is not a reason to park size on a brand-new rollup because the UI said "L2."

How It Actually Works

Optimistic rollups assume batches are honest and give people a window to challenge. Withdrawals to L1 wait out that window unless you use a fast bridge (which reintroduces a trust assumption). Arbitrum and Optimism are the household names.
ZK rollups post a validity proof. The math says the batch is correct. Withdrawals can be faster once the proof lands. The tradeoff is complexity and, often, a more centralized prover on day one.
Both need data availability. After 2024, that data often rides in blobs. If the data is hidden with a committee, you are closer to a validium than to a rollup. L2Beat grades this in public.

Risks I See in Sessions

A centralized sequencer can reorder or pause your transactions even when the rollup is "secured by Ethereum" for settlement. Those are different jobs.
Wrong-network sends. The address looks the same. The state machine is not. Test with dust.
Upgrade keys on the rollup contracts. Same lesson as a proxy and a timelock.

When It Matters

Choosing where to actually use Ethereum without paying mainnet gas for every click. The DeFi page is the wider frame. L2Beat is the scoreboard I send people to, not a Twitter thread.