Using the Standard Bridge


Certain interactions, like transferring ETH and ERC20 tokens between the two networks, are common enough that we've built the "Standard Bridge" to make moving these assets between L1 and L2 as easy as possible.

The standard bridge functionality provides a method for an ERC20 token to be deposited and locked on L1 in exchange of the same amount of an equivalent token on L2. This process is known as "bridging a token", e.g. depositing 100 USDC on L1 in exchange for 100 USDC on L2 and also the reverse - withdrawing 100 USDC on L2 in exchange for the same amount on L1. In addition to bridging tokens the standard bridge is also used for ETH.

The Standard Bridge is composed of two main contracts the L1StandardBridge (opens new window) (for Layer 1) and the L2StandardBridge (opens new window) (for Layer 2).

Here we'll go over the basics of using this bridge to move ERC20 assets between Layer 1 and Layer 2.

# Deposits

NOTICE

We're working hard to get more smart contract wallet software deployed and tested on Optimism. However, as a safety measure, we currently block smart contract wallets from calling the depositETH and depositERC20 functions. If you want to deposit using a smart contract wallet and you know what you're doing, you can use the depositETHTo and depositERC20To functions instead.

# Depositing ERC20s

ERC20 deposits into L2 can be triggered via the depositERC20 and depositERC20To functions on the L1StandardBridge (opens new window). You must approve the Standard Token Bridge to use the amount of tokens that you want to deposit or the deposit will fail.

Use the standard bridge contract only with standard bridge tokens

The standard bridge can only be used with tokens that have a properly configured ERC-20 version on Optimism. If you send any other type of token to the standard bridge directly (not using the user interface or the API), it gets stuck there and you lose that value.

Note that if you use the Optimism bridge UI (opens new window), or the Optimism SDK it automatically chooses the correct bridge contract, so this problem does not happen.

There are two ways to check if a token can use the standard bridge:

  1. Look in the token list (opens new window). If a token can use the standard bridge, then the "chainId": 10 entry will have the standard L2 bridge address, 0x4200..0010. For example, this entry shows that on the main Optimism network 0xBTC can use the standard bridge.

        {
       "chainId": 10,
       "address": "0xe0BB0D3DE8c10976511e5030cA403dBf4c25165B",
       "name": "0xBitcoin",
       "symbol": "0xBTC",
       "decimals": 8,
       "logoURI": "https://ethereum-optimism.github.io/data/0xBTC/logo.png",
       "extensions": {
         "optimismBridgeAddress": "0x4200000000000000000000000000000000000010"
       }
     },
    
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    In the token exists in the token list but does not use the standard bridge, the extensions.optimismBridgeAddress value is different. For example, this entry shows that on the main Optimism network DAI uses a different bridge:

        {
       "chainId": 10,
       "address": "0xDA10009cBd5D07dd0CeCc66161FC93D7c9000da1",
       "name": "Dai stable coin",
       "symbol": "DAI",
       "decimals": 18,
       "logoURI": "https://ethereum-optimism.github.io/data/DAI/logo.svg",
       "extensions": {
         "optimismBridgeAddress": "0x467194771dAe2967Aef3ECbEDD3Bf9a310C76C65"
       }
     },
    
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  2. You can "ask" the L2 token contract by calling it. ERC-20 tokens that can use the standard bridge not only if they:

    • Have an l2Bridge method
    • That method returns 0x4200...0010.

    For example, this link (opens new window) can be used to see that 0xBTC uses the standard bridge.

    Note that you cannot query the L1 token contract the same way. L2 contracts know the identity of their L1 counterpart, but L1 contracts only need to implement the standard ERC-20 methods.

# Depositing ETH

ETH deposits into L2 can be triggered via the depositETH and depositETHTo functions on the L1StandardBridge (opens new window). ETH deposits can alternatively be triggered by sending ETH directly to the L1StandardBridge. Once your deposit is detected and finalized on Optimism, your account will be funded with the corresponding amount of ETH on L2.

# Withdrawals

# Withdrawing ERC20s

ERC20 withdrawals can be triggered via the withdraw or withdrawTo functions on the L2StandardBridge (opens new window). If you'd like to see this contracts in action, you should check out the L1 ⇔ L2 deposit-and-withdraw example (opens new window).

# Withdrawing ETH

Unlike on L1, we do not have a separate function on L2 for withdrawing ETH. Instead, you can use the withdraw or withdrawTo functions on the L2StandardBridge (opens new window) and use the address 0xDeadDeAddeAddEAddeadDEaDDEAdDeaDDeAD0000 as the L2 token address.

# Adding an ERC20 token to the Standard Bridge

Tips

To add your token to the standard bridge, see the guide Adding an ERC20 token to the Standard Bridge (opens new window).

# The Optimism token list

The Standard bridge allows a one-to-many mapping between L1 and L2 tokens, meaning that there can be many Optimism implementations of an L1 token. However there is always a one-to-one mapping between L1 and L2 tokens in the Optimism token list (opens new window). The token list is used as the source of truth for the Optimism Gateway (opens new window) which is the main portal for moving assets between Layer 1 and Layer 2.

If you want to have your token added to the token list, you must make a pull request against the Optimism token list repository (opens new window). You'll need the addresses for both the L1 and L2 tokens, as well as a logo for the token. If you're looking for an example to follow, take a look at this simple pull request that adds a token to the token list (opens new window).