---
title: "Rollup node RPC"
description: "Reference for the Base rollup node's optimism_outputAtBlock method, which returns the L2 output root and sync metadata for a given block, along with the supporting BlockID, L1BlockRef, L2BlockRef, and SyncStatus structures."
source: https://basehub.org/api-reference/rollup-node-rpc/
---
Beyond the execution-layer JSON-RPC namespaces, the Base rollup node exposes its own RPC surface. The method documented here, `optimism_outputAtBlock`, is the one most integrators reach for: it hands back the 32-byte [L2 output root](/specifications/proofs-overview/) for a chosen block, which is the value bridges and proof systems anchor against.

## `optimism_outputAtBlock`

Given an L2 block number, this method returns that block's output root together with the metadata needed to reason about it.

### Parameters

| Name | Type | Description |
| :--- | :--- | :--- |
| `blockNumber` | `QUANTITY`, 64 bits | L2 integer block number |

### Returns

| Name | Type | Description |
| :--- | :--- | :--- |
| `version` | `DATA`, 32 Bytes | Output root version number, beginning with `0` |
| `outputRoot` | `DATA`, 32 Bytes | The output root |
| `blockRef` | `Object` | Instance of [`L2BlockRef`](#l2blockref) |
| `withdrawalStorageRoot` | `DATA`, 32 Bytes | Storage root of the `L2ToL1MessagePasser` contract |
| `stateRoot` | `DATA`, 32 Bytes | The state root |
| `syncStatus` | `Object` | Instance of [`SyncStatus`](#syncstatus) |

The input and output types follow the [engine API specification][engine-structures].

## Structures

The object types referenced by the rollup node API are listed below. They extend the structures defined in the [engine API specs][engine-structures].

### BlockID

- `hash`: `DATA`, 32 Bytes
- `number`: `QUANTITY`, 64 Bits

### L1BlockRef

- `hash`: `DATA`, 32 Bytes
- `number`: `QUANTITY`, 64 Bits
- `parentHash`: `DATA`, 32 Bytes
- `timestamp`: `QUANTITY`, 64 Bits

### L2BlockRef

- `hash`: `DATA`, 32 Bytes
- `number`: `QUANTITY`, 64 Bits
- `parentHash`: `DATA`, 32 Bytes
- `timestamp`: `QUANTITY`, 64 Bits
- `l1origin`: `BlockID`
- `sequenceNumber`: `QUANTITY`, 64 Bits - distance to first block of epoch

### SyncStatus

A point-in-time snapshot of the rollup driver's view of both chains.

- `current_l1`: `Object` - instance of [`L1BlockRef`](#l1blockref).
- `current_l1_finalized`: `Object` - instance of [`L1BlockRef`](#l1blockref).
- `head_l1`: `Object` - instance of [`L1BlockRef`](#l1blockref).
- `safe_l1`: `Object` - instance of [`L1BlockRef`](#l1blockref).
- `finalized_l1`: `Object` - instance of [`L1BlockRef`](#l1blockref).
- `unsafe_l2`: `Object` - instance of [`L2BlockRef`](#l2blockref).
- `safe_l2`: `Object` - instance of [`L2BlockRef`](#l2blockref).
- `finalized_l2`: `Object` - instance of [`L2BlockRef`](#l2blockref).
- `pending_safe_l2`: `Object` - instance of [`L2BlockRef`](#l2blockref).
- `queued_unsafe_l2`: `Object` - instance of [`L2BlockRef`](#l2blockref).

[engine-structures]: https://github.com/ethereum/execution-apis/blob/main/src/engine/paris.md#structures