How Does A Router Table Work, Types of Routing Table Entries

This article gives an overview of Routing Table, Components, How does a router table Work, Types of Routing Table Entries, Examples and more.

Routing Table

A routing table is a basic data structure or table that is kept in network equipment such as IP-enabled switches and routers. It serves as a “map” since it has the information and rules needed to route data packets across networks to their intended destinations. Every router has its own routing table, which is usually kept in the RAM of the device.

Purpose and Importance

The main function of a routing table is to help devices figure out the best or most efficient way to forward data packets to their destination. It is necessary for a router to forward packets.

Routing tables are crucial for several reasons, including:

Efficient Data Flow: They guarantee that data packets follow the most effective routes, enhancing network efficiency and cutting down on communication latency.

Network Communication: They make it possible for devices on several networks to successfully communicate.

Network Traffic Management: In order to control network traffic second-by-second and avoid congestion and data loss, routers employ routing tables.

Scalability Support: Routing tables can accommodate big, expanding networks with dynamic routing technologies.

Load Balancing: Divide traffic over several routes to boost efficiency and ease congestion.

Security Operations: For some security activities, such unicast reverse path forwarding (uRPF), where the router looks for a legitimate return route from the packet’s source address, they are essential.

How does a Router Table Work

A router looks at a data packet’s destination IP address after receiving it. Next, it looks up the most relevant entry that corresponds to this destination in its routing table. In order to forward the packet, the entry will specify the outgoing interface (the particular port on the router to transmit the packet over) or the next hop (the IP address of the next router or device in the path). Until the packet reaches its destination, each router along the path repeats this process.

Key Components of a Routing Table Entry

Usually, a routing table’s entries (or routes) have the following crucial fields:

Routing Protocol Code: A letter that represents the method of learning the route (e.g., ‘C’ for directly connected, ‘S’ for static, ‘R’ for RIP, ‘O’ for OSPF, and ‘D’ for EIGRP).

Destination Network / Network ID: The destination network or host’s IP address range or network ID is specified here.

Subnet Mask / Netmask: A 32-bit network address that indicates whether a host is a part of the local or remote network is also referred to as the netmask. Through the subnetting procedure, managers can apply a custom subnet mask to divide a network into two or smaller connected networks, improving routing performance and shrinking the broadcast domain.

Next Hop / Gateway: The next router or device on the route to the destination’s IP address. It provides the next forwarding destination for the packet.

Interface: Typically, routers link to external wide area networks (WANs) via serial interfaces and to other devices on the same network via Ethernet interfaces, such as eth0 or eth1. The device should use the inbound network interface, often referred to as the outgoing interface, listed in the routing table for forwarding the packet to the subsequent hop.

Metric / Cost: Each route that is accessible to a certain network is given a value by this entry. The value guarantees the router’s ability to select the optimal path. Sometimes the measure is the number of routers a packet of data must pass through in order to reach its destination. Priority is given to the path with the lowest metric when there are several paths to the same destination network.

Administrative Distance (AD): (Frequently found in tables with greater detail.) a number representing the reliability of a route discovered using a specific routing methodology. Lower AD values are ideal.

Route Flags: Indicate the route’s current state, such as whether it is operational.

Last Update Time: Shows how long it has been since the route was last updated.

Types of Routing Table Entries

Numerous route types can be found in routing tables, which can specify their particular purpose or how they were obtained:

Directly Connected Networks / Connected Routes: For active router interfaces, they are network route entries. A route entry for the network directly connected to a router is automatically generated when an IP address is configured on the router’s interface.

Static Routes: Manually set up by the network manager. Until they are manually updated, these entries remain fixed. Granular control is provided, however network changes necessitate manual upgrades.

Dynamic Routes: Automatically picked up knowledge from other routers when they used dynamic routing protocols (RIP, OSPF, EIGRP, BGP, ISIS, IGRP, etc.). These tables are dynamic and adjust to congestion, device malfunctions, and changes in the network.

Default Route / Gateway of Last Resort (0.0.0.0/0): When no particular entry in the routing table corresponds to the destination IP address, a special route is employed. The internet router is frequently the specified gateway to which it routes traffic. Its subnet mask is 0.0.0.0 at all times.

Host Routes: Extremely precise routes that define a path to a single IP address using a /32 mask.

Floating Static Routes: The administrative distance of static routes is higher than that of dynamically learnt routes. They are installed as backups solely in the event that the dynamic route of choice is unsuccessful.

How Routing Tables are Populated and Maintained

The router’s control plane is in charge of generating, preserving, and updating the routing table. This occurs mostly in three ways:

Directly Connected Networks: Inserted automatically when an interface is operational and has an IP address specified.

Static Routing: Manually set up by the network manager. They need to be manually updated if the network changes, but they are predictable.

Dynamic Routing Protocols: Learnt automatically from other routers that use BGP, OSPF, or RIP as routing protocols. These tables are dynamic and adjust to changes in the network.

Router Forwarding Decisions

When a router wants to forward a packet, it uses its routing database and a certain logic:

Longest Prefix Match: This is the main rule. The route with the most far-left bits matching (the most particular route, with the longest prefix length) is chosen by the router after comparing the packet’s destination IP address with all matching entries. If routes from different protocols are implemented, this rule takes precedence over the others, regardless of the administrative distance or metric.

Administrative Distance (AD): When several routes to the same location are discovered using various routing protocols, the router favours the one with the smallest administrative distance.

Metric: If the administrative distance of several routes from the same routing protocol is the same, the router selects the path with the lowest cost or metric.

In the event that no default route or matching route is detected, the router discards the packet and frequently returns an ICMP message to the source.

Challenges and Optimization

Routing loops, excessive memory and performance cost in large networks, and security flaws are some of the problems that might arise from using routing tables. Methods of optimisation include:

Route Summarization: Combines several networks into a single entry, decreasing the number of entries, increasing efficiency, and using less memory.

Load Balancing Implementation: Enables greater bandwidth utilization and congestion prevention by distributing traffic across several channels.

Route Filtering: Prevents superfluous information storage by controlling which routes are added to the table.

Quality of Service (QoS): Gives important network traffic top priority to ensure optimal performance.

Viewing the Routing Table

On Cisco routers, the most common command to view the IP routing table is show ip route. For IPv6, the command is show ipv6 route. You can also use variations like show ip route [connected | static | ospf] to display subsets of the routing table or show ip route ip-address for detailed information about a specific route. For host routing tables, the route print command can be used on operating systems like Windows.

Example of a Routing Table Entry

An abridged illustration of a routing table entry could look like this:

In this example:

• The first line shows a directly connected network (192.168.1.0) on interface eth0.
• The second line indicates a static route to the 10.0.0.0 network, forwarded via the next hop 192.168.1.1 on eth0.
• The third line shows a route to the 172.16.0.0 network learned via OSPF, with a metric of 110.
• The last line is the default route (0.0.0.0), directing all unmatchable traffic to 192.168.1.1.