Unit 3 · Level 2 · Layer 2s
Bridging L1 ↔ L2
Moving funds INTO a rollup via its official ('canonical') bridge is quick: lock on L1, appear on L2 within minutes. Moving OUT of an optimistic rollup is the slow part: the canonical exit waits out the roughly 7-day challenge window before funds unlock on Ethereum. Third-party bridges offer a shortcut (they front you the money on the other side for a fee), but now you're trusting them, not just the rollup.
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What you get asked
Order the steps of a canonical withdrawal from an optimistic rollup
The week of waiting is the security model doing its job, leaving time for fraud proofs before money leaves the rollup.
Why do optimistic rollup withdrawals take about a week?
Exits move real money off the rollup, so they get the full innocent-until-challenged treatment. ZK rollups can release much faster because validity is proven up front.
What's the real difference between a canonical bridge and a third-party bridge?
The canonical bridge is part of the rollup's own protocol. A third-party fast bridge is an extra business in the middle: convenient, but one more thing that can fail.
Fast third-party bridges front you the money on the other side, so you're now trusting the ___, not just the rollup.
Speed is bought with a new counterparty. Sometimes worth it, but know that's the trade you're making.
What's the practical takeaway before bridging anything?
Canonical or third-party, fast or slow, each route has a different trust bill. Unit 4 shows what happens when that bill comes due. 🐜
The rest of this unit
Rollups: do the work upstairs, settle the bill on Ethereum.