First steps

supersim allows testing multichain features locally. Previously, testing multichain features required complex docker setups or using a testnet.

To see it in practice, let's first try sending some ETH from the L1 to the L2.

Deposit ETH from the L1 into the L2 (L1 to L2 message passing)

1. Check initial balance on the L2 (chain 901)

Grab the balance of the sender account on L2:

cast balance 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266 --rpc-url http://127.0.0.1:9545

2. Send the Ether
It exists two different methods to do this action

First method - OptimismPortal

Send the Ether to OptimismPortal contract of the respective L2 (on chain 900)**

Initiate a bridge transaction on the L1:
In the case of the chain 901 the contract is 0x37a418800d0c812A9dE83Bc80e993A6b76511B57

cast send 0x37a418800d0c812A9dE83Bc80e993A6b76511B57 --value 0.1ether --rpc-url http://127.0.0.1:8545 --private-key 0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80

Second method - L1StandardBridge

Call bridgeETH function on the L1StandardBridgeProxy / L1StandardBridge contract of the respective L2 on L1 (chain 900) In the case of the chain 901 the contract is 0x8d515eb0e5F293B16B6bBCA8275c060bAe0056B0

Initiate a bridge transaction on the L1:

cast send 0x8d515eb0e5F293B16B6bBCA8275c060bAe0056B0 "bridgeETH(uint32 _minGasLimit, bytes calldata _extraData)" 50000 0x --value 0.1ether --rpc-url http://127.0.0.1:8545 --private-key 0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80

3. Check the balance on the L2 (chain 901)

Verify that the ETH balance of the sender has increased on the L2:

cast balance 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266 --rpc-url http://127.0.0.1:9545

With the steps above, you've now completed an L1 to L2 ETH bridge locally using supersim. This approach simplifies multichain testing, allowing you to focus on development without the need for complex setups or relying on external testnets.