Unit 3 · Level 2 · Layer 2s
Review: Layer 2s
Rollups execute upstairs and settle on Ethereum, splitting one L1 fee across thousands of users. Optimistic ones are innocent-until-challenged, guarded by fraud proofs and a ~7-day window; ZK ones arrive with validity proofs and skip the wait. Bridges connect the floors; canonical ones are slow but lean on the protocol itself. Sequencers remain powerful, mostly singular, and a work in progress. Review time.
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What you get asked
A rollup batch was posted, and for 7 days anyone may dispute it. Which design is this?
The challenge window is the signature move of optimistic rollups. ZK batches don't need one; their proof of correctness travels with them.
The challenge window on major optimistic rollups lasts about ___ days.
Roughly a week for fraud proofs to land, which is also why canonical withdrawals from these rollups test your patience.
Match each L2 term to its meaning
Four terms that cover most of any L2 conversation you'll ever have, or any L2 documentation you'll ever skim.
Why is a rollup safer than just some fast separate chain?
The L1 anchor is everything: data posted to Ethereum means the rollup's state can be rebuilt and exited even if its operators disappear.
Fees dropped from euros to cents when activity moved to L2s. What made that possible without gutting security?
Share the bill, keep the anchor. That one-two is the entire rollup thesis, and the reason Ethereum's lean-L1 bet from unit 2 pays off. 🐜
The rest of this unit
Rollups: do the work upstairs, settle the bill on Ethereum.