If you’re building or auditing smart contracts today, understanding where your code executes is just as important as what it does.
Let’s break down Layer 1s, Layer 2s, and Rollups from a technical and security lens 👇
⚙️ Layer 1 (L1)
Definition: The base blockchain — e.g. Ethereum, Solana, Bitcoin.
It handles consensus, data availability, and execution.
-
For devs: You deploy directly on the L1. Think
Ethereum mainnet. - For auditors: L1s have the strongest security guarantees — but execution is costly, and attack surfaces are well-known.
Examples: Ethereum, Solana, Avalanche, Bitcoin.
⚡ Layer 2 (L2)
Definition: A protocol built on top of an L1 to scale it — usually by moving execution off-chain and posting data or proofs back to the L1.
L2s inherit security from the L1, but optimize throughput and cost.
- For devs: You deploy on an L2 like Arbitrum, Optimism, or Base, using the same EVM tooling.
- For auditors: Key risks shift from on-chain contract logic to bridge security, sequencer assumptions, and proof verification.
🔁 Rollups
Rollups are the dominant design for L2s today.
They “roll up” many transactions, execute them off-chain, and post a single compressed proof or data batch to L1.
Two major flavors:
Optimistic Rollups — assume transactions are valid; fraud proofs can challenge invalid ones.
→ Examples: Optimism, Arbitrum.ZK-Rollups — use zero-knowledge proofs to prove correctness of batches.
→ Examples: zkSync, StarkNet, Scroll.
- For devs: Deployment often mirrors L1 contracts, but watch for subtle differences in gas accounting, precompiles, and bridging.
- For auditors: Verify data availability guarantees, proof system integrity, and bridge contracts — the weak link between layers.
🧩 Why It Matters
As a developer, you care about:
- Cost → L2s are cheaper.
- Throughput → Rollups scale better.
- Compatibility → Many L2s are EVM-equivalent.
As an auditor, you care about:
- Trust assumptions → Is the rollup fully decentralized?
- Upgradability → Who can pause or upgrade the bridge?
- Data security → Is transaction data actually available on L1?
🧠 TL;DR
| Layer | Executes | Posts Data To | Security From | Example |
|---|---|---|---|---|
| L1 | On-chain | Itself | Native consensus | Ethereum |
| L2 | Off-chain | L1 | L1 (via proofs) | Arbitrum, zkSync |
| Rollup | Off-chain batch | L1 | L1 + proof validity | Optimism, StarkNet |
🔍 Final Thought
L1s are about security.
L2s are about scalability.
Rollups are about trust minimization.
When developing or auditing across these layers, know your execution context — that’s where most hidden risks live.
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