What Is Feasible Successor in Networking?

Imagine you drive to work every day and your usual route goes over a bridge. One morning, the bridge is closed for repairs. If you know another road that works just as well and you have that route saved in your memory, you can switch to it immediately without stopping to check a map.

That backup route is your feasible successor. In computer networks, routers use a protocol called EIGRP to find the best path to send data. The best path is called the successor, and the feasible successor is a backup path that meets certain criteria.

The main requirement is that the backup path must be guaranteed not to create a loop, meaning it is a safe, reliable alternative. The router keeps a list of these backup routes ready. If the main path fails, the router switches to the feasible successor without any delay, keeping the network running smoothly.

This is important because networks need to be reliable, and having a quick backup helps avoid interruptions. Without this feature, the router would have to recalculate all possible paths, which takes time and can slow things down. The feasible successor makes the network more resilient and efficient, which is why it is a key concept for networking professionals to understand.

A Feasible Successor is a concept integral to the Enhanced Interior Gateway Routing Protocol (EIGRP), a Cisco proprietary distance-vector routing protocol. EIGRP uses the Diffusing Update Algorithm (DUAL) to maintain loop-free paths and provide rapid convergence. The successor is the primary route to a destination, chosen based on the lowest feasible distance (FD).

The FD is the metric value calculated for a path, typically based on bandwidth, delay, load, reliability, and maximum transmission unit (MTU), though by default only bandwidth and delay are used. A feasible successor is a backup route that is guaranteed to be loop-free. For a route to qualify as a feasible successor, it must satisfy the feasibility condition: the reported distance (RD) from the next-hop router must be less than the current feasible distance (FD) of the successor.

The RD is the metric advertised by the neighbor for that destination, representing the neighbor's own distance to the target. This condition ensures that the backup path does not route traffic back through the local router, preventing routing loops. When a successor fails, EIGRP immediately promotes a feasible successor (if one exists) to become the new successor, without needing to run DUAL and send queries to neighbors.

This provides sub-second convergence in many cases. If no feasible successor exists, EIGRP enters an active state and sends queries to all neighbors to find an alternative path. The concept of feasible successor is implemented in the EIGRP topology table, which stores all known routes to a network, including both successors and feasible successors.

The topology table is distinct from the routing table, which contains only the best routes. In real IT implementations, network engineers rely on feasible successors to design highly available networks, especially in WAN or data center environments where link failures can have significant impact. Feasible successors also influence EIGRP load balancing; by default, EIGRP can load balance across up to four equal-cost paths, but it can also be configured for unequal-cost load balancing using the variance command, which allows traffic to be shared across feasible successors with different metrics as long as they meet certain criteria.

Understanding feasible successors is critical for troubleshooting EIGRP convergence issues and for optimizing network performance.

In practical IT networking, the concept of feasible successor is essential for building reliable and fast-converging networks. When a network link goes down, every second of downtime can cost an organization money, productivity, and reputation. EIGRP's ability to use feasible successors allows routers to switch to backup paths in milliseconds, often without any noticeable interruption to users.

This is especially important in critical environments like financial trading, healthcare systems, or emergency services where network availability is paramount. Network professionals must understand feasible successors to design redundant topologies correctly. For example, if a network has multiple paths to a remote site, engineers need to ensure that at least one path qualifies as a feasible successor to achieve fast failover.

If the design does not satisfy the feasibility condition, the router will have to fall back to the slower query process, increasing convergence time. Additionally, feasible successors play a role in unequal-cost load balancing. By using the variance command, administrators can allow traffic to be distributed across multiple paths, including feasible successors, to utilize network bandwidth more efficiently.

This can improve performance without buying more circuits. However, misconfiguration can lead to suboptimal routing or even loops. Therefore, a deep understanding of feasible successors helps network engineers avoid mistakes and optimize their EIGRP deployments.

In summary, feasible successors are not just a theoretical concept; they are a practical tool that directly impacts network performance and reliability. For anyone studying for the CCNA exam, mastering this topic is crucial because it appears in multiple exam objectives and real-world troubleshooting scenarios.

In the CCNA exam, feasible successor questions often appear in multiple-choice, drag-and-drop, and simulation formats. One typical pattern is a scenario-based question that presents a network diagram with multiple EIGRP routers. The question describes a failure of the primary link and asks which backup route will be used immediately.

The answer depends on whether any path meets the feasibility condition. Another common question displays a topology table listing multiple routes with their Feasible Distance (FD) and Advertised Distance (AD). The candidate must identify which entries are successors and which are feasible successors.

For example, the topology table might show Route A: FD 100, AD 50; Route B: FD 120, AD 90; Route C: FD 200, AD 150. If the successor has FD 100, then Route B with AD 90 is a feasible successor because 90 < 100. Route C with AD 150 is not a feasible successor because 150 is not less than 100.

There are also troubleshooting questions where a router fails to converge quickly because no feasible successor exists. The candidate must determine the cause, such as unequal metrics due to different bandwidths or delays on parallel links. Another pattern involves the variance command.

For instance, 'An EIGRP network has a successor with metric 1000 and two feasible successors with metrics 2000 and 2500. If variance is set to 3, which routes will be used for load balancing?' The candidate must know that variance multiplies the successor's metric by the variance value, and any feasible successor with a metric less than that product is eligible.

So, 1000 * 3 = 3000, so both feasible successors qualify. Simulation questions might ask to configure EIGRP and then verify feasible successors using 'show ip eigrp topology'. The candidate might be asked to adjust an interface metric to turn a non-feasible path into a feasible successor.

Understanding these question patterns helps candidates focus their study on the practical application of the feasibility condition and EIGRP convergence behavior.

Let us dive deeper into the practical aspects of feasible successors in EIGRP. As a network professional, you will often need to design and troubleshoot EIGRP networks. The first step is understanding how to influence the feasibility condition through link metrics.

By default, EIGRP uses bandwidth and delay. Bandwidth is inversely proportional to the metric; a higher bandwidth results in a lower cost. Delay is additive. For example, if you have two parallel links to a neighbor, one with 100 Mbps and the other with 10 Mbps, the 100 Mbps link will produce a lower FD.

If the path through the 10 Mbps link has an AD that is less than the FD of the 100 Mbps link, it becomes a feasible successor. But if the AD is too large, it does not. In practice, to ensure fast failover, you can configure the delay on backup links to be low, or adjust bandwidths, but careful planning is needed because these changes also affect path selection.

Another key point is the use of the 'variance' command for unequal-cost load balancing. Variance allows traffic to be distributed across multiple paths, including feasible successors, even if their metrics are higher. For instance, if the successor has a metric of 100 and you set variance to 2, any feasible successor with a metric up to 200 can be used for load balancing.

This can improve throughput, but you must ensure that the paths are loop-free (which they are, because they are feasible successors). However, using variance without proper planning can cause suboptimal routing if the backup path has much higher delay or loss. When troubleshooting, you should use 'show ip eigrp topology' to view the topology table.

In the output, feasible successors are indicated by the state code 'P' (Passive) and the number of successors (e.g., 1 successor, 0 feasible successors). If the output shows '0 feasible successors', it means the network lacks a loop-free backup.

You can then examine the AD values to see why certain routes do not qualify. For example, if a route has AD 120 and the successor's FD is 100, that route fails the condition. You might then adjust the bandwidth or delay on the backup link to lower its AD, perhaps by increasing the bandwidth or decreasing the delay.

But be careful: increasing bandwidth on a slow link does not speed it up; it only changes how the router calculates the metric. In practice, you should set bandwidth to match the actual speed of the link. One common mistake is using 'bandwidth' command on a serial link to trick EIGRP, but this can lead to routing inconsistencies.

Instead, adjust the delay using 'delay' command on the interface, which is a more straightforward way to tweak metrics. Remember that changing delay affects the path's metric for all routes using that interface. In summary, mastery of feasible successors requires not only theoretical knowledge but also hands-on practice with configurations and show commands.

Practice on lab equipment or simulators to see how changes in metrics affect the topology table and convergence. This practical experience will serve you well in both the exam and real-world networking.