> ## Documentation Index > Fetch the complete documentation index at: https://docs.polygon.technology/llms.txt > Use this file to discover all available pages before exploring further. # Polygon PoS Architecture > Architectural overview of Polygon PoS from a node perspective, covering the Heimdall-v2 consensus layer and Bor execution layer. Polygon PoS is structured as a two-layer system: a consensus layer called Heimdall-v2 and an execution layer called Bor. Nodes on Polygon are designed around this two-layer split. Bor handles block production; Heimdall-v2 handles validation, checkpoint submission, and coordination with Ethereum. Figure: Ethereum, Bor and Heimdall architecture ## Three-layer view The network can be described as three layers: * **Ethereum layer**: a set of contracts on Ethereum mainnet. These include staking contracts, checkpoint storage, and the bridge. * **Heimdall-v2 layer**: a set of proof-of-stake nodes running in parallel to Ethereum mainnet. Heimdall-v2 monitors staking contracts on Ethereum, validates Bor block data, and submits checkpoints to Ethereum. Built on Cosmos SDK and CometBFT. * **Bor layer**: a set of block-producing nodes shuffled by Heimdall. Built on Go Ethereum. ## Staking contracts on Ethereum The PoS mechanism relies on staking management contracts deployed on Ethereum mainnet. These contracts: * Allow anyone to stake POL tokens and register as a validator. * Distribute staking rewards for validating state transitions on the network. * Record checkpoints submitted by Heimdall. The PoS mechanism also provides a partial mitigation for the data unavailability problem on Polygon sidechains, since checkpoint data is anchored to Ethereum. ## Heimdall-v2: validation layer Heimdall-v2 aggregates blocks produced by Bor into Merkle trees and publishes the Merkle root periodically to Ethereum. These periodic snapshots are called checkpoints. For every set of Bor blocks, a Heimdall-v2 validator: 1. Validates all blocks since the last checkpoint. 2. Creates a Merkle tree of the block hashes. 3. Publishes the Merkle root hash to Ethereum mainnet. Checkpoints serve two purposes: * Providing finality on Ethereum for cross-chain withdrawals. * Providing proof of burn for asset withdrawal to Ethereum. Block producer selection works as follows: * A subset of active validators from the pool is selected as block producers for a span. These producers create and broadcast blocks. * A checkpoint includes the Merkle root hash of all blocks in a given interval. All nodes validate the hash and attach their signatures. * A proposer from the validator set collects all signatures and submits the checkpoint to Ethereum mainnet. * Validator selection probability is proportional to stake ratio in the overall pool. Heimdall-v2 is a complete rewrite of the original Heimdall. It is based on Cosmos SDK and CometBFT. The migration is specified in: * [PIP-43: Replacing Tendermint with CometBFT](https://github.com/maticnetwork/Polygon-Improvement-Proposals/blob/cb371136414b5e198c44750cd4c30f7aad16043a/PIPs/PIP-43.md) * [PIP-44: Upgrade Cosmos-SDK](https://github.com/maticnetwork/Polygon-Improvement-Proposals/blob/cb371136414b5e198c44750cd4c30f7aad16043a/PIPs/PIP-44.md) * [PIP-62: Heimdall-v2 Migration](https://github.com/maticnetwork/Polygon-Improvement-Proposals/blob/cb371136414b5e198c44750cd4c30f7aad16043a/PIPs/PIP-62.md) Throughout these docs, "Heimdall" refers to Heimdall-v2 unless otherwise specified. ## Bor: block production layer Bor is Polygon PoS's block producer, responsible for aggregating transactions into blocks. Block producers are a rotating subset of validators selected periodically by Heimdall based on stake. See [Bor architecture](/pos/architecture/bor/introduction/) for detailed mechanics.