What is a layer 2?
Ethereum processes a limited number of transactions per block. When demand is high, users bid against each other and gas fees rise. A layer 2 (L2) is a separate network that executes transactions on its own, then settles them on Ethereum, the layer 1 (L1).
Picture a highway with an express lane alongside it. Most traffic moves in the express lane, and only a summary of it merges back onto the highway. Thousands of L2 transactions share the cost of one Ethereum transaction, so each one pays a fraction.
How rollups work
The most common L2 design is the rollup. A sequencer collects transactions, orders and executes them, then posts the transaction data and the resulting state to Ethereum in batches. Because the data is on Ethereum, anyone can rebuild the L2 state and check it.
Optimistic rollups
Batches are assumed valid. Anyone can challenge a wrong one during a dispute window of about a week, which is why withdrawals through the official bridge take that long.
Examples: Arbitrum, Base, OP Mainnet
ZK rollups
Each batch comes with a cryptographic validity proof that Ethereum checks. No dispute window, so withdrawals finalize once the proof is accepted, at the cost of heavy proving infrastructure.
Examples: Linea, zkSync Era, Starknet, Scroll
Networks you can swap on with ChainBridge
ChainBridge compares 0x, KyberSwap and ParaSwap on Ethereum and on the networks below, plus BNB Chain and Avalanche.
| Network | Chain ID | Type | Withdrawal to Ethereum | Notes |
|---|---|---|---|---|
| Arbitrum One | 42161 | Optimistic rollup | About 7 days via the official bridge | Deep DeFi liquidity; all ChainBridge sources available |
| Base | 8453 | Optimistic rollup (OP Stack) | About 7 days via the official bridge | Run by Coinbase, easy to fund from its exchange |
| OP Mainnet (Optimism) | 10 | Optimistic rollup (OP Stack) | About 7 days via the official bridge | Shares the OP Stack with Base and other chains |
| Linea | 59144 | ZK rollup | Hours, once a validity proof is accepted | On ChainBridge: 0x and KyberSwap, no gasless swaps |
| Polygon PoS | 137 | Sidechain, not a rollup | Depends on its own validators and bridge | Own validator set; checkpoints to Ethereum but does not inherit its security |
Third-party bridges and cross-chain swaps get you off an optimistic rollup in minutes instead of a week: a liquidity provider pays you on the other side and waits out the window itself, for a fee.
What changes for your swaps
Lower gas
A swap that costs several dollars on Ethereum mainnet typically costs cents on a rollup, since the L1 cost is shared across a whole batch. Fees still rise when the L2 itself is busy.
Faster confirmation
The sequencer confirms in about a second or two, against roughly 12 seconds per block on Ethereum. Full finality on Ethereum comes later, when the batch settles.
Separate liquidity
Each network has its own pools. A pair that is deep on Ethereum can be thin on a smaller L2, so check price impact on large trades.
Different tokens
USDC on Base is a different contract from USDC on Arbitrum, and some chains have both native and bridged versions. Check the address.
Choosing a network for a trade
On mainnet, gas is a fixed cost per transaction, so it hurts small trades most. A $5 network fee is 5% of a $100 swap but 0.05% of a $10,000 one (example figures).
- Small and frequent trades: an L2 such as Arbitrum or Base. Gas will rarely matter.
- Large trades: compare. Mainnet often has the deepest liquidity, which can save more on price impact than you pay in gas. The quote shows both.
- Funds already on a network: trade there. Moving them costs a bridge fee and time.
More on fees in Understanding gas fees.
Moving funds to a layer 2
Withdraw directly if you can
Many exchanges withdraw straight to Arbitrum, Base or Optimism. This skips bridging entirely; pick the network carefully in the withdrawal form.
Or swap across chains
Select a different destination network in the ChainBridge swap. Routes from Relay and LI.FI are compared on what you receive, with any fee shown in the quote. See Cross-chain trading.
Keep some gas on arrival
Most of these networks use ETH for gas; Polygon uses POL. Make sure you arrive with a little, or receive some as part of the transfer.
Bridges are among the most attacked contracts in crypto. Read Bridge security before moving large amounts.