What is a Layer 2 blockchain?
A Layer 2 (L2) blockchain is a separate network that sits on top of a Layer 1 and processes transactions off the base chain, then settles back to it for final security and settlement. By handling most of the computational work off-chain, Layer 2s can process far more transactions per second at much lower cost than the underlying L1, without requiring changes to the L1 itself.
Most Layer 2 networks are built on Ethereum. Well-known examples include Arbitrum, Optimism, Base, zkSync, and Starknet. PYUSD launched on Arbitrum in July 2025, making it the first major stablecoin to expand to an Ethereum L2 as a primary deployment.
Why Layer 2 exists
Layer 1 blockchains like Ethereum face a fundamental throughput constraint. Ethereum processes around 15 to 30 transactions per second at the base layer. When demand spikes, gas fees rise steeply and the network becomes congested. Changing the L1 itself to be faster risks compromising its security or decentralization.
Layer 2 solves this by moving most activity off the main chain. The L1 handles final settlement and security. The L2 handles execution at scale. The result is a system where thousands of transactions per second are possible without weakening the security guarantees of the base chain.
How Layer 2 rollups work
The dominant L2 architecture is the rollup. A rollup batches many transactions together, processes them off-chain, compresses them, and posts a summary or proof back to the L1. There are two main types.
Optimistic rollups
Optimistic rollups assume all transactions in a batch are valid unless someone challenges them. After a batch is posted to the L1, there is a challenge window, typically seven days, during which anyone can submit a fraud proof if they believe a transaction was invalid.
If no challenge is raised within the window, the batch is finalized. The seven-day window is also why withdrawing funds from an optimistic rollup back to Ethereum mainnet takes up to a week, unless a fast bridge is used.
Examples: Arbitrum, Optimism, Base.
ZK rollups
ZK rollups (zero-knowledge rollups) use cryptographic validity proofs to verify each batch before it is posted to the L1. Rather than assuming correctness and allowing challenges, ZK rollups mathematically prove the validity of every transaction in the batch upfront. This enables faster finality but is computationally more expensive to generate.
Examples: zkSync, Starknet, Polygon zkEVM.
Both types inherit Ethereum's security because the L1 validates the proofs or data posted by the L2. If the L2 were to act dishonestly, the L1 would reject the invalid state.
EIP-4844 and the cost reduction
In March 2024, Ethereum's Dencun upgrade introduced EIP-4844, also known as proto-danksharding. This added a new type of data storage called blobs to Ethereum blocks, specifically designed for rollup data. Because blobs are cheaper to store than standard calldata and are pruned after a short period, the cost for L2s to post their batch data to Ethereum fell by over 90% almost immediately after the upgrade. Transaction fees on major L2s dropped to fractions of a cent as a result.
Layer 2 and stablecoin payments
Layer 2 networks are increasingly relevant for stablecoin payment infrastructure. USDC and USDT are available natively on several L2s, and the fee and speed profile of L2 transactions makes them practical for high-frequency payment flows that would be prohibitively expensive on Ethereum mainnet.
There are two practical considerations for payment use cases specifically:
- Finality timing: Transactions achieve fast local finality on the L2 within seconds, but batch finality on the L1 typically takes several minutes. Hard finality for optimistic rollups can take up to seven days. For most payment applications, L2 local finality is sufficient, but this distinction matters for settlement-critical use cases
- Travel Rule compliance: L2 rollup batching compresses multiple transactions into a single L1 submission, which can complicate sender and receiver identity verification for cross-border transfers subject to Travel Rule requirements. Stablecoin orchestration platforms handle this by maintaining transaction-level data separately from the on-chain batch
Layer 2 vs. sidechain
These terms are sometimes confused. A sidechain is a separate blockchain with its own security model that bridges assets to and from an L1. If the sidechain's validators act dishonestly, funds on the sidechain can be at risk independently of the L1.
A true Layer 2 derives its security from the L1 through proofs or data posted on-chain. The distinction matters: an L2 user is ultimately protected by Ethereum's security. A sidechain user is protected only by the sidechain's own security model, which is typically weaker.
