How a layer 2 works
An L2 runs its own chain, collects transactions into batches and posts them to Ethereum. Anyone can check the batches against Ethereum’s record, which is where the security comes from.[1]
ethereum.org gives one test for whether a network extends Ethereum: whether it stores its data on Ethereum mainnet. Projects that do are called rollups. If the data lives somewhere else, the project is independent rather than a direct Ethereum extension. There is also no official L2; anyone can build one.[1]
Rollups post that data either as calldata or in blobs. The Fusaka upgrade in December 2025 added PeerDAS, a more efficient way for L2s to post and retrieve data on Ethereum.[1]
Types of layer 2
Optimistic rollups assume batches are valid and allow a challenge period for fraud proofs. Zero-knowledge (zk) rollups post a cryptographic proof with each batch.[2]
In an optimistic rollup, anyone can challenge a batch during the challenge period by computing a fraud proof. If the challenge succeeds, the rollup re-executes the transactions, corrects its state and penalizes the sequencer that included the bad transaction.[3]
In a zk rollup, a smart contract on Ethereum holds the rollup's state and only updates it after verifying a validity proof. Because the proof finalizes the state, exits have no challenge delay: withdrawals execute once the contract verifies the proof.[4]
Examples by chain ID from chainid.network: OP Mainnet 10, Base 8453 and Arbitrum One 42161 (optimistic); zkSync 324, Linea 59144 and Scroll 534352 (zk).[7]
Using a layer 2
You add the network to your wallet with its chain ID and an RPC endpoint, bridge funds from Ethereum and pay gas fees in the L2’s token, usually ETH.
Risks
In an optimistic rollup, a centralized operator processes transactions and submits blocks to Ethereum, and ethereum.org notes that operators can censor users. Withdrawals from optimistic rollups through the official bridge take roughly seven days because of the challenge period.[3]
How blobs cut L2 fees
EIP-4844 added a transaction type (0x03) that carries blobs. A blob is 4,096 field elements × 32 bytes = 131,072 bytes, or 128 KiB, and costs 2^17 = 131,072 blob gas.[5]
EIP-4844 originally set the target at 393,216 blob gas per block and the maximum at 786,432. Divided by 131,072 per blob, that is a target of 3 blobs and a maximum of 6 per block. Blob gas has its own base fee with a minimum of 1 wei.[5]
EIP-7691, activated with the Pectra upgrade, raised the blob target to 6 and the maximum to 9 per block.[9]
Blobs are not kept forever. Nodes must serve them for 4,096 epochs, which the EIP puts at about 18 days. That is long enough for anyone to check the L2's data, but much cheaper than storing it permanently.[5]
Illustrative cost split: if a rollup fits compressed transfers of about 100 bytes each into one blob, 131,072 ÷ 100 = 1,310 transfers share one blob. If posting that blob cost 0.001 ETH, each transfer's share is 0.001 ÷ 1,310 ≈ 0.00000076 ETH. A plain transfer on Ethereum at an illustrative 10 gwei costs 21,000 × 10 = 210,000 gwei = 0.00021 ETH, roughly 275 times more.[8]
Example: withdrawing from an optimistic rollup
The dates here are illustrative. You start a withdrawal from an optimistic rollup through its official bridge on September 1. Your request goes into the next batch posted to Ethereum.[3]
The challenge period lasts roughly seven days, so you can complete the claim on Ethereum around September 8. The same withdrawal from a zk rollup can finish as soon as the batch's validity proof is verified.[3]
How to check an L2 yourself
L2BEAT rates rollups by stage. When checked in September 2026 it listed Base and Arbitrum One at Stage 1, described as limited training wheels: users can exit in the presence of malicious operators even if the Security Council disappears. Stage 0 is labeled full training wheels. L2BEAT notes that stages do not reflect project security.[6]
ethereum.org's layer 2 FAQ says the safety of each network depends on its technology, smart contract security and maturity, and suggests starting with small transactions.[1]
The Bottom Line
An L2 gives you Ethereum-based security at a fraction of mainnet fees. Check which bridge you use and how the network is run.
Educational content, not financial advice.