Routing Table

Routing Table is a data structure used by routers to store and manage route information. It determines the best path for forwarding packets to their destination. Each entry in a routing table corresponds to a specific destination network and includes information about the next hop, metrics, and routing protocol.

Overview

The routing table is a fundamental component of the control plane in a network. It is built and maintained by routing protocols or through manual configuration. Routers use the routing table to make forwarding decisions and update the forwarding table in the data plane.

Structure

A typical routing table contains the following fields:

• Destination Network: The IP address range or subnet for the route.
• Subnet Mask: Specifies the size of the destination network.
• Next Hop: The IP address of the next router or device along the path.
• Metric: A value representing the cost of the route (e.g., hop count, delay, bandwidth).
• Interface: The local network interface to use for forwarding packets.
• Routing Protocol: Indicates the protocol (e.g., OSPF, BGP) that added the route.

Example Routing Table:

Destination Network	Subnet Mask	Next Hop	Metric	Interface	Protocol
192.168.1.0	255.255.255.0	10.0.0.1	1	eth0	OSPF
10.0.0.0	255.0.0.0	10.0.1.1	10	eth1	BGP
0.0.0.0	0.0.0.0	192.168.1.254	5	eth0	Static

Types of Routes

Routing tables include different types of routes based on how they are learned:

• Static Routes: Manually configured by network administrators.
• Dynamic Routes: Automatically learned and updated using routing protocols (e.g., RIP, OSPF, BGP).
• Default Routes: Used when no specific match is found in the routing table; often represented as 0.0.0.0/0.
• Connected Routes: Directly connected networks that the router is aware of.

How It Works

1. When a packet arrives, the router examines its destination IP address.
2. The router searches the routing table for the most specific matching route using techniques like Longest Prefix Matching.
3. Once a match is found, the packet is forwarded to the next hop or local interface specified in the route.

Dynamic Routing Protocols

Routing tables are dynamically populated and maintained by various protocols:

• RIP (Routing Information Protocol): Uses hop count as a metric.
• OSPF (Open Shortest Path First): Calculates the shortest path using Dijkstra's algorithm.
• BGP (Border Gateway Protocol): Exchanges routing information across autonomous systems.
• EIGRP (Enhanced Interior Gateway Routing Protocol): Uses a composite metric of bandwidth and delay.

Applications

• Enterprise Networks: Routing tables guide traffic between different subnets and external networks.
• Service Providers: Manage large-scale routing across wide-area networks.
• Data Centers: Used to ensure efficient routing in complex, multi-tier architectures.

Advantages

• Provides efficient and automated routing decisions.
• Adapts dynamically to network changes using routing protocols.
• Scalable for networks of varying sizes and complexities.

Limitations

• Large routing tables can consume significant memory and CPU resources.
• Incorrect routing table entries can lead to routing loops or blackholes.
• Static routes require manual updates, which can be error-prone in dynamic networks.

See Also

• Forwarding Table
• Longest Prefix Matching
• Routing Protocols
• Control Plane
• Data Plane