Unit 3 · Level 2 · Layer 2s
Rollup intuition
Imagine paying a card fee on every single drink at a bar. Absurd. You run a tab and settle once. Rollups apply the same trick to Ethereum: execute hundreds of transactions on a faster layer above, compress them into one batch, and post that batch down to the L1. The expensive base chain is touched once instead of hundreds of times, yet it still anchors the final record.
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What you get asked
What's the core idea of a rollup?
Execution moves up to the cheap, fast layer; settlement and data stay anchored to Ethereum. Split the job, keep the security.
Follow one transaction through a rollup
Fast for you at the top, one bulk receipt at the bottom. The batch on L1 is what makes the rollup more than a fast database.
Why are rollup transactions so much cheaper than L1 transactions?
One batch, one L1 fee, thousands of payers. Your share of the bar tab is tiny because the whole bar is on it.
A rollup posts compressed transaction ___ to Ethereum, which is how it inherits the L1's security.
Because the data lands on Ethereum, anyone can reconstruct the rollup's state from the L1, even if the rollup's own operators vanish.
What exactly does a rollup inherit from Ethereum?
The rollup executes; Ethereum holds the record and the exits. That anchor is the difference between a Layer 2 and just another chain. 🐜
The rest of this unit
Rollups: do the work upstairs, settle the bill on Ethereum.