What is an Ethereum Layer 2? Rollups, blobs and what “secure” really means

Ethereum settles roughly a dozen transactions a second on its base layer, and when demand spikes the fee to use it can exceed what the transaction is worth. Rather than change the base chain to go faster, which would make it harder to run and verify, the Ethereum ecosystem chose to scale by building networks on top of it. These Layer 2s, or L2s, now process the large majority of Ethereum-related activity, hold tens of billions of dollars in assets, and are where most new applications, including Coinbase’s Base, launch. Understanding what they are and what security they actually provide is necessary for anyone assessing Ethereum as infrastructure.

The rollup model

A rollup is a chain of its own with its own blocks and its own transaction fees, usually a fraction of a cent. Users deposit assets by locking them in a smart contract on Ethereum, which mints a matching balance on the L2. Transactions then happen on the L2, batched by an operator called a sequencer. Periodically the sequencer posts the batch’s data, compressed, back to Ethereum, along with a commitment to the resulting state. Ethereum stores that data, and because it does, anyone can reconstruct the L2’s state from Ethereum alone. That is what “inherits Ethereum’s security” means: the L2’s history cannot be lost or rewritten as long as Ethereum stands, and users can in principle withdraw their assets through the Ethereum contract even if the L2’s operators disappear.

The two rollup families differ in how Ethereum knows the posted state is correct. An optimistic rollup assumes it is and opens a challenge window, typically seven days, during which anyone can submit a fraud proof showing a transaction was invalid. If no one does, the state is final; this is why withdrawals from Arbitrum or Optimism to Ethereum take a week unless a third party fronts the liquidity. A zero-knowledge rollup submits a cryptographic validity proof with each batch, which Ethereum verifies in a few milliseconds; finality is near immediate and withdrawals are fast, at the cost of generating proofs, which was expensive but has fallen sharply as the technology matured.

Why 2024 was the turning point

Until March 2024, rollups posted their data into ordinary Ethereum transaction space and competed with everyone else for it, which kept L2 fees at tens of cents. The Dencun upgrade introduced EIP-4844, or proto-danksharding, which created a separate, cheap data channel called blobs that Ethereum stores for about 18 days, long enough for anyone to verify the rollup but not forever. L2 fees dropped by more than 90 percent within days. Transaction counts on Base, Arbitrum and the others rose to many multiples of Ethereum’s own, and the base chain’s role settled into being the settlement and data availability layer for the networks above it. Subsequent upgrades have raised the number of blobs per block to keep pace with demand.

The landscape

By activity and assets the leaders are Arbitrum One, Coinbase’s Base, and OP Mainnet, all optimistic rollups; Base in particular became the busiest network in the ecosystem by transactions after its 2023 launch and is Coinbase’s on-chain strategy in a single product. zkSync Era, Starknet, Linea, Scroll and Polygon’s zkEVM lead the zero-knowledge group. Optimism’s OP Stack and Arbitrum’s Orbit turned rollup software into a kit, so new chains, including ones from Kraken, Sony and various DeFi protocols, can launch as L2s in weeks. A related category, validiums and other designs that keep data off Ethereum, trades some of the security guarantee for lower cost and is generally treated as a weaker form.

What “secure” actually means for an L2

L2BEAT, the ecosystem’s reference monitor, grades rollups on a scale of Stage 0 to Stage 2 that is worth understanding. Stage 0 means the chain runs rollup software but its security still depends on the operators: the fraud or validity proof system may be incomplete, and a multisig held by the team can upgrade the contracts instantly. Stage 1 requires a working proof system and a security council with a supermajority to upgrade. Stage 2 means the system is fully enforced by code with only a long delay for upgrades. As of 2025 only a small number of large rollups had reached Stage 1, and almost none Stage 2. The practical consequences are that on most L2s the sequencer is a single server run by the team (it can censor or halt, though users can usually force transactions through Ethereum after a delay), and a compromised upgrade key could, in theory, move user funds.

For an institution the checklist is therefore: which stage is the chain at, who holds the upgrade keys and with what delay, is there a forced-exit mechanism that works without the sequencer, how long do withdrawals take and who is fronting the liquidity for fast ones, and where does the chain post its data. Throughput numbers matter less than any of those.

The economics

Rollups collect fees from users and pay Ethereum for blob space, keeping the difference; Base has generated meaningful revenue for Coinbase on that spread. Most also have a token, used for governance and sometimes to pay the sequencer, though the largest, Base, does not. The open question in the ecosystem is whether value accrues to the L2s, which capture the activity, or to Ethereum, which sells them settlement and data; the fee cuts from Dencun sharply reduced what rollups pay the base chain, and that debate shapes how investors think about ether itself.