Layer 1 vs. layer 2 blockchains: a layer 2 review primer

A layer 1 blockchain is a base network that records and finalizes its own transactions, such as Bitcoin or Ethereum. A layer 2 is a system built on top of a layer 1 that processes most transactions off the base chain and reports the results back to it periodically. The goal is to lower fees and raise capacity while relying on the base chain for settlement and security.

This layer 2 review primer explains the vocabulary, how the main designs work, why the difference between a rollup and a sidechain matters, and what to check before relying on any of them. It draws on analysis from the Bank for International Settlements (BIS) and the Financial Stability Board (FSB), and it does not recommend any network or token.

In brief

• Layer 1 networks have their own validators and consensus rules. Layer 2 networks depend on a layer 1.
• Rollups bundle many transactions off-chain and post a batch or proof to layer 1, so they inherit its security. Sidechains do not.
• Scaling comes with trade-offs, including fragmented liquidity, reliance on operators and added complexity.
• Moving assets between layers is a distinct risk worth reading about before using it.

What the layers are

Layer 1

A layer 1 is the base blockchain. It has its own nodes, its own consensus mechanism and its own token for paying fees. Transactions settle directly on it. The BIS appendix on blockchain consensus rates layer 1 designs approximately on decentralization, security and scalability, and frames the choice as the blockchain trilemma: it is hard to maximize all three at once. Bitcoin’s proof-of-work design, for example, is rated high on decentralization and security but low on scalability, with blocks about every ten minutes.

When many people want to transact at the same time, a layer 1 with limited capacity becomes congested. Users compete to be included, and fees rise. That pressure is the main reason layer 2 designs exist.

Layer 2

The BIS describes a layer 2 as a solution in which most transactions occur off-chain and are only periodically reported to layer 1. The base chain acts as a court of final record and does not process every transaction itself. Because many users share the cost of posting data to layer 1, the cost per transaction falls.

The main layer 2 designs

Rollups

A rollup bundles transactions off-chain and posts a batch, or a proof about the batch, to layer 1. The BIS notes that rollups inherit the security of the base layer. There are two principal types.

• Optimistic rollups assume that batches are valid and rely on fraud proofs during a challenge window. The security model assumes that at least one honest participant is watching and willing to challenge an invalid batch. The challenge window means withdrawals back to layer 1 may take time.
• Zero-knowledge rollups use validity proofs, which are cryptographic proofs that the batch was computed correctly. The BIS describes them as offering fast finality without a challenge period.

Validiums

A validium also uses validity proofs but keeps transaction data off-chain, relying on a data availability committee instead of the base layer. The BIS lists this design separately because it gives up some of the security of a rollup in exchange for lower cost. If the committee withholds data, users may be unable to reconstruct their balances.

Sidechains

A sidechain is an independent blockchain with its own validators that connects to a layer 1 through a bridge. The BIS is clear that sidechains run independently and do not inherit layer 1 security. A sidechain is only as secure as its own validator set, which may be small. Calling a sidechain a layer 2 is common in marketing, but the security properties are different, which is why the distinction matters in any layer 2 review.

A worked example

The following example is hypothetical.

Suppose a base chain can process a fixed number of transactions in each block, and during a busy period a simple token transfer costs a user a few dollars in fees. A rollup operator collects 1,000 user transfers, computes the new balances off-chain, and posts a compressed record of the batch to the base chain, along with either a validity proof or a fraud-proof window.

The base chain pays attention to one posted batch instead of 1,000 separate transfers. The fee for posting is split among the 1,000 users, so each pays a small fraction of what a direct transaction would cost. If the rollup operator tried to post a false balance, the proof would fail, or in an optimistic design a watcher could submit a fraud proof during the challenge window, and the false batch would be reversed.

Now consider a user who wants to leave. In a zero-knowledge rollup, the withdrawal can be processed once the proof is accepted. In an optimistic rollup, the user may need to wait through the challenge window unless a third party offers a faster exit for a fee. In a sidechain, the user depends on the bridge and the sidechain’s validators to release the funds.

Trade-offs to understand

Security

The central question is where final security comes from. For a rollup, the answer is the base layer, subject to the correctness of the rollup’s contracts and the availability of its data. For a validium, it is split between proofs and an off-chain committee. For a sidechain, it is the sidechain’s own validators.

Centralization of operators

Many layer 2 networks rely on a small number of operators, often called sequencers, to order transactions. If a sequencer fails or acts against users, transactions can be delayed or censored. Some networks provide a mechanism to force transactions through the base layer, and others are developing decentralized sequencing. An analyst should read the documentation to see what exists today, not what is planned.

Fragmentation

The BIS appendix notes that layer 2 and sidechain solutions can improve scalability but tend to fragment liquidity and execution environments. Ethereum, for example, scales by shifting execution to layer 2 rollups. The result is that assets and users are spread across many networks, and moving between them takes steps and fees. The FSB has also noted in its work on decentralized finance that composability links applications, so risks on one network can spread to others.

Bridges

Moving assets between a layer 1 and a layer 2, or between two layer 2 networks, usually involves a bridge, which is a set of contracts and sometimes operators that lock assets on one side and issue them on the other. A bridge is a concentrated point of risk, because a flaw in its code can affect many users at once. Check how a bridge is secured and whether it has been audited, and remember that an audit does not guarantee safety.

Upgrade keys

Some layer 2 networks are controlled by administrators who can upgrade the contracts. That can allow quick fixes, but it also means a small group may be able to change the rules. Look for information on who holds upgrade authority and whether there are delays or limits on its use.

Layer 1 vs. layer 2 at a glance

• Settlement: layer 1 finalizes its own transactions. Layer 2 posts results to a layer 1.
• Security: layer 1 relies on its own validators. A rollup relies on the base layer. A sidechain relies on its own validators.
• Cost: layer 2 fees are typically lower because costs are shared across many users, though they vary with demand.
• Speed: layer 2 can confirm quickly for users, but final settlement on layer 1 may take longer.
• Complexity: layer 2 adds bridges, operators and extra software that layer 1 users do not face.

Common misconceptions

A layer 2 is not simply a faster copy of a layer 1. It has a different trust model, and the details differ for each design.

Lower fees do not mean lower risk. The risk has moved from congestion to smart contract, operator and bridge considerations.

A layer 2 token is not necessarily the source of the network’s security. In a rollup, security comes from the base layer’s validators and the proof system, while the layer 2’s own token may play a separate role in governance or fees. Read the documentation to see what the token does.

Not every network that calls itself a layer 2 meets the BIS definition. Check whether the network posts data and proofs to a base layer and whether users can exit without the operator’s cooperation.

What to check in a layer 2 review

1. Which base layer does it settle on, and what exactly is posted there?
2. Is it a rollup, a validium or a sidechain, and where is transaction data stored?
3. How do proofs or fraud challenges work, and who can submit them?
4. Who runs the sequencer, and can users force a transaction through the base layer?
5. Who controls upgrades, and is there a delay?
6. How do withdrawals work, and how long do they take?
7. How concentrated are the assets and the users?

Official investor resources, such as the SEC’s investor alert on crypto asset securities, remind readers that crypto asset markets can be volatile and illiquid, and that platforms can fail or freeze withdrawals. This guide is educational and is not investment advice.

FAQ

Is Ethereum a layer 1 or a layer 2?

Ethereum is a layer 1. The BIS notes that it scales by shifting execution to layer 2 rollups, which post their results back to Ethereum.

Are rollups as secure as the base chain?

Rollups inherit the base chain’s security for what is posted there, according to the BIS. Their safety also depends on the correctness of their contracts, the proof system, data availability and the operators.

Why are some layer 2 withdrawals slow?

Optimistic rollups use a challenge window so that fraud proofs can be submitted. A withdrawal to the base layer generally has to wait until that window closes, unless a third party provides a faster exit for a fee.

What is the difference between a rollup and a sidechain?

A rollup posts transaction data or proofs to the base layer and inherits its security. A sidechain runs with its own validators and does not inherit that security.

Do layer 2 networks have their own tokens?

Some do and some do not. Where a token exists, its role varies. It may pay fees, support governance or serve other purposes, and it does not by itself secure a rollup. Check the project’s documentation for what the token actually does.