Layer 2 (L2) solutions have largely solved the congestion issues of the Ethereum Layer 1 (L1) mainnet, yet transferring assets between L2s has emerged as a new pain point. If you want to move funds from Arbitrum to Optimism, traditional cross-chain bridges often take 5-15 minutes and incur significant costs, resulting in a user experience far inferior to same-chain transactions. Across was born for this specific scenario—Relayers advance the funds to you on the destination chain, then recoup their costs once the source chain transaction reaches finality. For the user, the entire process takes anywhere from a few seconds to a few minutes. Before starting, ensure your accounts are ready: register at the Binance Official Site and complete KYC (exchanges serve as a safety net if a bridge fails); for the app, use the Binance Official App; for iPhone users, refer to the iOS Installation Guide. Below is a deep dive into Across's mechanisms and operations.
I. Why is Across So Fast?
The traditional bridge workflow involves: Locking funds on the source chain → Waiting for finality → Relaying the message → Minting on the destination chain. The "waiting for finality" part on L2 can take 5-30 minutes because it must account for the challenge period on L1.
Across skips this step:
- The user initiates a cross-chain request on the source chain;
- A Relayer immediately uses their own liquidity to pay the user on the destination chain (usually arriving within 1-2 minutes);
- After the source chain transaction is finalized, the Across protocol reimburses the Relayer from its liquidity pool;
- The "Optimistic Verification" is handled by the UMA protocol for dispute resolution.
The risk lies with the Relayer, while the user enjoys "near-instant" arrival.
II. Comparison with Other Bridges
| Dimension | Across | Stargate | cBridge |
|---|---|---|---|
| L2 Transfer Speed | 1-3 minutes | 3-8 minutes | 5-10 minutes |
| L2 Transfer Fee | ~0.04% | ~0.06% | ~0.04% + base |
| Supported Chains | Ethereum + Major L2s (~10) | ~12 chains | 40+ chains |
| Supported Assets | ETH, USDC, USDT, DAI, WBTC, etc. | Stablecoins + ETH | Many |
| Failure Protection | Auto-refund on failure | Refund | Refund / cfAsset |
Conclusion: For cross-chain transfers of mainstream assets between L2s, Across is almost the default choice.
III. Step-by-Step: Arbitrum USDC → Optimism
Preparation
- Wallet: MetaMask, Rabby, or Coinbase Wallet;
- Reserve 0.0005 ETH on Arbitrum for gas;
- Gas isn't strictly required on Optimism beforehand (you'll receive USDC, then you can bridge a small amount of ETH back via Across or bridge ETH directly);
- USDC ≥ 1 (the entry threshold is extremely low).
Steps
- Open across.to and verify the URL spelling (be wary of phishing sites using similar terms like "acros");
- Connect your wallet, set "From" to Arbitrum and "To" to Optimism, with USDC as the asset;
- Enter the amount; the interface will display the "Estimated time" and "Fees";
- If it's your first time bridging this asset, you'll need to "Approve" it;
- Sign the confirmation; a progress bar will appear on the page;
- Funds arrive within 1-3 minutes and are immediately usable on the destination chain.
Handling Large Transfers
The single-transaction limit for Across depends on the remaining liquidity in the destination chain's pool. When bridging over $50,000, the interface may show a "Slow path" warning—this means Relayers are unwilling to advance such a large amount, and the transaction will follow the traditional flow, slowing down to 30-60 minutes. If you see this, either split the transfer into smaller batches or use an exchange as an intermediary.
IV. Fee Breakdown
A typical 100 USDC transfer from Arbitrum to Optimism costs:
- Across LP fee: ~0.04 USDC (dynamic, based on pool utilization);
- Relayer fee: ~0.05 USDC (covers gas on Optimism);
- Source chain gas (Arbitrum): ~$0.05;
Total: Approximately 0.1-0.15 USDC, making it one of the cheapest L2-to-L2 transfer solutions currently available.
V. Bridging ETH is Even More Cost-Effective
When bridging ETH, Across doesn't need to perform a swap; it simply transfers within the LP pools, making the protocol fee nearly zero (leaving only the source chain gas). If you just need to move ETH from Arbitrum to Optimism, Across has virtually no competition.
VI. Potential Risks
While Across has operated for over two years without major incidents, keep the following in mind:
- Optimistic Verification Hypothesis: Relies on UMA's Optimistic Oracle. If someone submits a false claim and no one challenges it, funds could theoretically be lost. In practice, economic incentives make the cost of attack much higher than the potential gain;
- Relayer Liquidity: During peak times or for large amounts, Relayers might not advance funds, leading to a degraded "slow path" experience;
- Smart Contract Vulnerabilities: A risk shared by all bridges; consider splitting very large amounts;
- Phishing: Phishing sites are common in search engine ads; always use a verified bookmark.
FAQ
Q: Does Across support BNB Chain? A: No. Across primarily serves Ethereum and its L2 ecosystem. To bridge to BNB Chain, use Stargate or cBridge.
Q: What happens if a cross-chain transfer fails? A: Across has an automatic refund mechanism. If the source chain transaction rolls back, your funds remain at the original address. You can view the status of transactions that haven't reached finality on the across.to/transactions page.
Q: Can I bridge non-integer amounts? A: Yes, down to the token's smallest unit (e.g., 6 decimal places for USDC).
Further Reading
- For non-EVM chains or BNB Chain, use Stargate;
- For niche or minor chains, choose cBridge;
- If you're uncomfortable with bridges, use Exchange Intermediation.
Across handles L2-to-L2 transfers exceptionally well. We recommend daily DeFi users keep it as a primary tool in their workflow.