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
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
- 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
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
- 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
- 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
- Ensures efficient and accurate packet delivery.
- Reduces complexity by abstracting real-time forwarding decisions.
- Supports scalability by maintaining only active paths and routes.
Limitations
- 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.