Forwarding Table
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Forwarding Table is a data structure used by network devices, such as routers and switches, to determine the next hop or output interface for forwarding packets. It plays a crucial role in the data plane, enabling efficient and accurate packet forwarding based on precomputed rules.
Overview[edit | edit source]
The forwarding table maps packet header information, such as destination IP addresses or MAC addresses, to specific output ports or next-hop devices. It is typically built using information from the routing table (in the control plane) and other protocols, such as ARP (Address Resolution Protocol) or VLAN tagging.
Key Features[edit | edit source]
- High-Speed Lookup: Forwarding tables are optimized for rapid lookup operations to meet real-time data forwarding requirements.
- Compact Representation: They use efficient data structures, such as trie-based trees or hash tables, to minimize memory usage.
- Dynamic Updates: Forwarding tables are updated dynamically as network conditions change, such as when routes are added or removed.
Structure[edit | edit source]
A typical forwarding table entry includes:
- Destination Address: The destination IP or MAC address of the packet.
- Prefix Length: In IP forwarding, this indicates the subnet or network range.
- Next Hop: The address or identifier of the next device in the path.
- Output Interface: The network interface through which the packet should be forwarded.
- Metric (Optional): A value indicating the cost of the route, used for decision-making in multi-path scenarios.
Example Forwarding Table:
| Destination Network | Prefix Length | Next Hop | Output Interface |
|---|---|---|---|
| 192.168.1.0 | 24 | 10.0.0.1 | eth0 |
| 10.0.0.0 | 16 | 10.0.1.1 | eth1 |
| 0.0.0.0 | 0 | 10.0.2.1 | eth2 |
Relationship to Routing Table[edit | edit source]
- The routing table exists in the control plane and stores all potential routes learned from routing protocols or static configurations.
- The forwarding table is derived from the routing table and is used in the data plane for real-time packet forwarding.
- Forwarding tables are smaller and optimized for lookups compared to routing tables, which store more extensive information.
Applications[edit | edit source]
- Routers: Forwarding tables guide IP packets to the appropriate next hop in a routed network.
- Switches: Used for MAC address forwarding in Layer 2 networks.
- SDN Devices: Forwarding tables are dynamically programmed by the SDN controller to implement specific traffic policies.
Advantages[edit | edit source]
- Ensures efficient and accurate packet delivery.
- Reduces complexity by abstracting real-time forwarding decisions.
- Supports scalability by maintaining only active paths and routes.
Limitations[edit | edit source]
- Requires constant synchronization with the routing table to handle network changes.
- High-speed lookups can be resource-intensive in large-scale networks.
- Forwarding tables are specific to the data plane and cannot make high-level routing decisions.
