What is a Layer 2?

Ethereum can process about 15–30 transactions per second on its base layer. For a global financial platform, that is far too slow. Layer 2 (L2) solutions solve this scalability problem by processing transactions off the main chain while still inheriting Ethereum's security. This guide explains the different types of Layer 2 solutions, compares the major L2 networks, and explores how they will shape the future of Ethereum.

The Blockchain Scalability Trilemma

The scalability trilemma, coined by Vitalik Buterin, states that a blockchain can only optimize for two out of three properties simultaneously:

• Decentralization: Anyone can run a node and validate transactions without requiring specialized hardware.
• Security: The network is resistant to attacks and censorship.
• Scalability: The network can handle a high volume of transactions at low cost.

Ethereum chose to prioritize decentralization and security at the base layer, which limits its throughput. Layer 2 solutions break the trilemma by adding scalability on top of a secure and decentralized L1. The L1 provides the security foundation, while the L2 provides the execution speed.

What are Layer 2 Solutions?

Layer 2 solutions are protocols built on top of a base blockchain (Layer 1) that process transactions off-chain while leveraging the L1 for security and finality. They work by batching many transactions together, executing them in a faster environment, and then posting a compressed summary or proof back to the main chain.

The key property of a true L2 (as opposed to a sidechain) is that users can always withdraw their funds to L1 even if the L2 operators become malicious or go offline. This is called the escape hatch or forced withdrawal mechanism.

Types of Layer 2 Solutions

Optimistic Rollups

Optimistic rollups assume all transactions are valid by default ("optimistic") and only run computation in case of a dispute. A sequencer batches transactions, executes them, and posts the resulting state root along with the compressed transaction data to Ethereum L1.

After the state root is posted, there is a challenge period (typically 7 days) during which anyone can submit a fraud proof if they believe the state root is incorrect. If a fraud proof succeeds, the invalid state transition is reverted and the malicious sequencer is penalized.

Optimistic Rollup Flow:
1. Users submit transactions to the L2 sequencer
2. Sequencer batches and executes transactions off-chain
3. Sequencer posts state root + compressed data to L1
4. 7-day challenge period begins
5. If no valid fraud proof → state is finalized
6. If fraud proof succeeds → state is reverted

Major Optimistic Rollups include Arbitrum, Optimism, and Base.

ZK Rollups

ZK (Zero-Knowledge) rollups use cryptographic validity proofs to verify every batch of transactions before posting to L1. Instead of assuming transactions are valid and waiting for disputes, ZK rollups mathematically prove that the new state is correct.

ZK Rollup Flow:
1. Users submit transactions to the L2 sequencer
2. Sequencer batches and executes transactions off-chain
3. A prover generates a validity proof (ZK-SNARK or ZK-STARK)
4. Sequencer posts state root + validity proof to L1
5. L1 smart contract verifies the proof → state is finalized immediately

The main advantage of ZK rollups is instant finality on L1 (no 7-day challenge period). The tradeoff is that generating ZK proofs is computationally expensive, and achieving full EVM equivalence with ZK circuits is technically challenging.

Major ZK Rollups include zkSync Era, Polygon zkEVM, Scroll, Linea, and StarkNet.

State Channels

State channels allow participants to conduct multiple transactions off-chain by locking funds in a multisig contract on L1 and exchanging signed messages. Only the opening and closing transactions are posted on-chain. This is best suited for applications with a fixed set of participants, such as payment channels or turn-based games.

Bitcoin's Lightning Network is the most prominent example of state channels in production. On Ethereum, state channels have been largely superseded by rollups, which offer more general-purpose scalability.

Sidechains

Sidechains are independent blockchains with their own consensus mechanisms that run parallel to the main chain. They are connected to L1 via a bridge. Unlike true L2s, sidechains do not derive their security from Ethereum.

Optimistic Rollups vs ZK Rollups

Major Layer 2 Networks Compared

Here is a comparison of the most prominent Ethereum L2 networks as of early 2026:

How Rollups Work

At their core, rollups work by moving transaction execution off Ethereum while keeping transaction data on Ethereum. Here is a more detailed look at the process:

The Sequencer

The sequencer is the L2 entity responsible for ordering and executing transactions. When you submit a transaction to an L2, it goes to the sequencer, which:

1. Receives and orders incoming transactions
2. Executes transactions against the current L2 state
3. Produces L2 blocks with the resulting state changes
4. Batches multiple L2 blocks together for efficiency
5. Compresses the batch data and submits it to L1

Currently, most L2s use a centralized sequencer operated by the L2 team. This is a known centralization risk that also has implications for MEV extraction, and most L2 teams have roadmaps to decentralize sequencing over time.

Data Availability

A critical property of rollups is that they post transaction data to L1. This ensures that anyone can reconstruct the L2 state from L1 data alone, which is necessary for fraud proofs (Optimistic) and for users to withdraw their funds independently.

Before EIP-4844, rollups posted data in calldata, which is expensive because it is stored permanently by all Ethereum nodes. After EIP-4844, rollups can use blobs, which are cheaper because they are pruned after ~18 days.

Bridge Mechanics

Bridges are the mechanisms that transfer assets between L1 and L2. Each rollup has a canonical bridge contract deployed on Ethereum that handles deposits and withdrawals:

Deposits (L1 to L2)

1. User sends tokens to the L2 bridge contract on L1.
2. The bridge contract locks the tokens on L1.
3. The L2 detects the deposit event on L1.
4. The L2 mints equivalent tokens to the user's L2 address.

Deposits are typically fast (minutes) because the L2 only needs to observe a confirmed L1 transaction.

Withdrawals (L2 to L1)

1. User initiates a withdrawal on L2.
2. The L2 burns the tokens and includes the withdrawal in the next batch.
3. Optimistic Rollups: Wait for the 7-day challenge period to pass, then claim tokens on L1.
4. ZK Rollups: Wait for the validity proof to be generated and verified on L1, then claim tokens. This is usually minutes to hours.

Gas Savings: L1 vs L2

The primary benefit of L2s for users is dramatically lower transaction costs. Here is a comparison of typical costs:

These cost reductions are the result of amortization: the fixed cost of posting data to L1 is spread across hundreds or thousands of transactions in each batch. Calculate exact gas costs for your transactions with our Gas Fee Calculator.

EIP-4844: Proto-Danksharding and Blobs

EIP-4844, activated in March 2024 with the Dencun hard fork, was a landmark upgrade for L2 scalability. It introduced a new transaction type that carries blobs — large chunks of data (~128 KB each) specifically designed for rollup data.

Key improvements from EIP-4844:

• Cheaper data: Blob data is priced separately from regular calldata with its own fee market, reducing L2 data costs by 10–100x.
• Temporary storage: Blobs are pruned after ~18 days, so they do not permanently bloat the Ethereum state.
• Separate fee market: Blob fees operate independently from execution gas fees, preventing L2 costs from spiking during L1 congestion.
• Future-proof: EIP-4844 is a stepping stone to full Danksharding, which will further increase blob capacity.

The result has been transformative: L2 fees dropped dramatically after the Dencun upgrade. Many L2s now offer sub-cent transactions for simple operations. Learn more about how gas fees work and the relationship between L1 and L2 costs in our EIP-1559 Explained guide.

Frequently Asked Questions