feat(book): Genesis - Rollup Config

book/src/building/rollup-configs.md

@@ -1,3 +1,38 @@
 # Rollup Configs
 
+Rollup configurations are a consensus construct used to configure an Optimism Consensus client.
+When an OP Stack chain is deployed into production or consensus nodes are configured to sync the chain,
+certain consensus parameters can be configured. These parameters are defined in the [OP Stack specs][config].
 
+Consensus parameters are consumed by OP Stack software through the `RollupConfig` type defined in the
+[`op-alloy-genesis`][genesis] crate.
+
+## `RollupConfig` Type
+
+The [`RollupConfig`][rc] type is defined in [`op-alloy-genesis`][genesis].
+
+A predefined rollup config can be loaded from a given L2 chain id using
+the [`rollup_config_from_chain_id`][rcid] method. An example is shown below.
+
+```rust
+use op_alloy_genesis::{OP_MAINNET_CONFIG, rollup_config_from_chain_id};
+
+let op_mainnet_config = rollup_config_from_chain_id(10).expect("infallible");
+assert_eq!(OP_MAINNET_CONFIG, op_mainnet_config);
+```
+
+The `OP_MAINNET_CONFIG` is one of the predefined rollup configs exported by
+the [`op-alloy-genesis`][genesis] crate. Other predefined configs include
+the following.
+
+- `OP_MAINNET_CONFIG`
+- `OP_SEPOLIA_CONFIG`
+- `BASE_MAINNET_CONFIG`
+- `BASE_SEPOLIA_CONFIG`
+
+<!-- Links -->
+
+[rcid]: https://docs.rs/op-alloy-genesis/latest/op_alloy_genesis/rollup/fn.rollup_config_from_chain_id.html
+[rc]: https://docs.rs/op-alloy-genesis/latest/op_alloy_genesis/rollup/struct.RollupConfig.html
+[genesis]: https://crates.io/crates/op-alloy-genesis
+[config]: https://specs.optimism.io/protocol/configurability.html