Layer 2 Explained: Rollups, Blobs, Fees and Bridge Risks
Lesson 10 of 10 · Beginner
Why did layer 2 become necessary?
Space in every Ethereum block is limited. When many users want to transact at once, they compete for room in the block and fees rise. In busy periods, a simple trade on a decentralised exchange on the main layer could become uneconomic for small amounts.
The solution the Ethereum ecosystem chose was to keep the main layer (layer 1 or L1) as the “final court” and the security layer, and move most transaction execution to separate networks called layer 2 (L2). A layer 2 executes transactions quickly and cheaply, then records a summary or their data on the main layer so that it benefits from its security. Arbitrum, Optimism (OP) and Base are well-known examples. From a user's point of view, a layer 2 is a separate network in the wallet, with similar-looking 0x addresses but usually much lower fees.
How does a rollup work? Optimistic and ZK
The most common type of layer 2 is the rollup: it bundles hundreds or thousands of transactions (rolls them up) and publishes a compressed package of their data on Ethereum. There are two main ways to prove those packages are correct:
- Optimistic rollup: assumes correctness and sets a “challenge period” in which anyone can reject an incorrect package by submitting a fraud proof. That is why withdrawing to the main layer through these rollups' official bridges usually takes about a week.
- ZK rollup: produces a mathematical proof (a validity proof) for each package showing that it is correct. It needs no long challenge period, but the technology is more complex.
An important fact: most rollups still have centralised components, for example a sequencer that orders transactions, or a council that can upgrade the contracts. The common “stages” framework (Stage 0 to 2) measures how decentralised a rollup is. According to The Block's 2026 outlook, Arbitrum, OP and Base were at Stage 1, and no major rollup had reached Stage 2. The same report put the combined share of Arbitrum and Base in layer-2 DeFi total value locked at about 77 percent.
Why fees fell after blobs
The biggest cost of a rollup is publishing its data on Ethereum. Before 2024, rollups recorded this data in ordinary transaction space, which was expensive. With the Dencun upgrade in 2024, a new kind of data space called a blob was added: data that is kept only for a limited time and has its own fee market. Because rollups only need the data to be available for a while so that anyone can check it, blobs were enough for them and far cheaper. The result was a sharp fall in transaction fees on layer 2s.
The Fusaka upgrade, which according to the official Ethereum blog went live on 3 December 2025, increased blob capacity further with PeerDAS technology. One realistic caveat: low fees are not permanent. If demand for blobs exceeds capacity, the blob fee market becomes expensive too.
What is a bridge, and what is a bridged token?
Networks naturally do not know about each other. A bridge is a mechanism that “moves” an asset between two networks. The common method is lock and mint:
- You lock the asset in the bridge contract on the source network.
- The bridge sends a message to the destination network saying “this amount was locked”.
- On the destination network, an equivalent wrapped token is minted for you.
- To return, the destination token is burned and the original asset is released.
So a bridged token is really a receipt: its value depends on the original asset actually being locked in the bridge contract and on messages being verified correctly. A rollup's official bridge relies on the security of that rollup itself, but third-party bridges have a separate trust layer: a set of verifiers, signers or oracles.
Why bridges get hacked so often
A bridge contract usually holds a large amount of locked assets in one place, and all of it depends on a single “message” being correct. If an attacker can pass off a forged message as valid, they can mint tokens without locking anything or release the locked assets. Two recent examples:
- Kelp DAO, April 2026: according to CoinDesk and Chainalysis, after a developer's computer was compromised, a forged cross-chain message passed a configuration that had only one verifier, and about 292 million dollars was lost. The effects spread to other protocols, and about 5.4 billion dollars left the restaking sector.
- Liquid Network, September 2026: according to TRM Labs, about 319 million dollars of bitcoin left this Bitcoin sidechain because of a bug in the verifiers' software, and about 85 percent of it was later returned.
Practical lessons: every use of a bridge means accepting that bridge's risk; keep the number of hops between networks small; send a small amount the first time; make sure the token you receive on the destination is the widely accepted version on that network; and never use a bridge link from a message or an ad, because fake bridge sites are common.
Common mistakes
- Assuming every network called “layer 2” has exactly Ethereum's security; sequencers and upgrade keys are separate risks.
- Sending assets to your address on a layer 2 that the destination wallet or exchange does not support.
- Treating a bridged token as identical to the original asset; its value depends on the health of the bridge.
- Being surprised by the roughly one-week withdrawal time through the official bridge of optimistic rollups.
- Using a bridge link from an ad or an anonymous message instead of the official address.
- Passing assets through several bridges in a row to save a small amount in fees.
What do volume and data say here?
For layer 2s and bridges, public data helps you gauge risk yourself instead of relying on marketing:
- The bridge contract in an explorer: you can see the balance of the bridge contract on the source network and compare it with the supply of the bridged token on the destination network. The two should line up; a large gap is a sign of trouble.
- Concentration of activity: when a large part of the locked value sits in one or two layer 2s (about 77 percent in Arbitrum and Base, according to The Block), liquidity and tooling are deeper there, but concentration risk is also greater.
- Independent layer 2 risk dashboards: these show each rollup's decentralisation stage, whether it has fraud or validity proofs, who holds the upgrade keys and its withdrawal time. Read those columns before holding a significant amount on a network.
- Actual fees: in each layer 2's explorer you can see the fees your transactions paid; comparing them with the main layer gives a real picture of the savings.
This data is for gauging infrastructure risk; none of it is a signal to buy or sell any network's token.
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Frequently asked questions
What is an Ethereum layer 2?
It is a separate network that executes transactions quickly and cheaply and records their data or proofs on Ethereum to benefit from the main layer's security. Arbitrum, Optimism and Base are well-known examples.
Why does withdrawing from some layer 2s take a week?
On optimistic rollups, a withdrawal through the official bridge must pass a challenge period so that an incorrect package can be rejected. That period is usually about a week. Third-party services are faster but carry their own risk.
Is using a bridge safe?
No bridge is risk-free. Bridges hold a lot of assets in one place and have been the target of major attacks many times. Test with a small amount, use the official address and keep the number of bridges small.
Education only, not a recommendation to buy or sell; we never give signals. Price examples are illustrative. Test any method on a demo account first, with strict risk management.