VMware ESXi and vSphere Cluster Management

EIGRP Reported Distance and Feasible Distance Explained

Learn how EIGRP reported distance, advertised distance, and feasible distance work, how to read FD/RD topology output, and how successors and feasible successors are selected.

EIGRP uses several pieces of information to choose routes and maintain backup paths. Two of the most important values are reported distance (RD) and feasible distance (FD). They describe different parts of a route to the same destination, viewed from different routers.

This lesson explains how to interpret these values in the EIGRP topology table, how they affect successor selection, and how they support loop-free backup-route selection.

What EIGRP Distance Values Represent

EIGRP, or Enhanced Interior Gateway Routing Protocol, is a dynamic routing protocol that uses composite metrics. Under the default metric weights, bandwidth and delay are the primary components of that metric. EIGRP maintains neighbor, topology, and routing-table information.

When a router learns a route from an EIGRP neighbor, it receives metric information about the neighbor's path to the destination. The receiving router then combines that information with the cost of reaching the neighbor.

Therefore, the two distance values describe separate portions of one end-to-end path:

  • Reported distance: the neighbor's metric for reaching the destination.
  • Feasible distance: the local router's complete metric for reaching the destination through that neighbor.

These values are maintained per destination. The same prefix can have different RD and FD values at different routers because each router has a different position in the network and a different cost to its neighbors.

Reported Distance and Advertised Distance

Reported Distance (RD) is the EIGRP metric that a neighboring router reports for reaching a particular destination. From the receiving router's viewpoint, RD represents the neighbor-to-destination portion of the route.

Advertised Distance (AD) is another name for reported distance in EIGRP. In this context, “advertised” means the metric a neighbor advertises for its own route to the destination.

For example, if R2 tells R1 that R2 can reach 192.168.0.0/24 with an EIGRP metric of 28160, R1 records 28160 as the reported distance received from R2.

Feasible Distance

Feasible Distance (FD) is the complete EIGRP metric calculated by the local router for reaching a destination through a particular neighbor.

Conceptually, the local router calculates:

local path metric = cost to the neighbor + neighbor's reported metric

The actual EIGRP calculation uses the composite metric and its configured components, such as bandwidth and delay. The displayed values should not be treated as arbitrary numbers that can always be added directly. Interface characteristics and EIGRP metric calculation rules determine the resulting value.

When multiple EIGRP paths exist, the path with the lowest feasible distance is normally preferred. That preferred path is called the successor, and its next hop is normally installed in the IP routing table.

Relationship Between FD and RD

FD is the local router's total distance to the destination. RD is the next-hop neighbor's declared remaining distance to that destination.

  • RD begins at the neighboring router's view of the route.
  • FD begins at the local router and covers the entire path through that neighbor.
  • FD normally exceeds RD because the local router must also account for the cost of reaching the neighbor.

The values are related, but they are not interchangeable. RD is received from the neighbor, while FD is calculated by the local router for that particular path.

Reading FD/RD Notation in the Topology Table

EIGRP topology output commonly displays a metric pair in the form:

FD/RD
  • The first number is the local feasible distance for the path.
  • The second number is the neighbor's reported distance, also called advertised distance.

For example:

30720/28160

This means the local router calculated a feasible distance of 30720 through that neighbor, and the neighbor reported a distance of 28160 to the destination.

Reported Distance vs. Feasible Distance

Attribute: Whose perspective the value represents
Reported/Advertised Distance: The neighboring router's perspective
Feasible Distance: The local router's perspective

Attribute: What portion of the route is measured
Reported/Advertised Distance: The neighbor-to-destination portion
Feasible Distance: The complete local-router-to-destination path

Attribute: Where the value originates
Reported/Advertised Distance: Supplied by the neighbor
Feasible Distance: Calculated by the local router

Attribute: How it appears in topology output
Reported/Advertised Distance: The second value in FD/RD notation
Feasible Distance: The first value in FD/RD notation

Attribute: Role in successor selection
Reported/Advertised Distance: Describes the neighbor's path but does not by itself select the best path
Feasible Distance: The lowest FD normally identifies the successor

Attribute: Role in feasible-successor eligibility
Reported/Advertised Distance: Compared with the successor's FD
Feasible Distance: Provides the current successor threshold for that comparison

Worked Two-Router Example

Consider two EIGRP neighbors:

  • R1 and R2 form an EIGRP adjacency.
  • R2 is directly connected to the 192.168.0.0/24 destination LAN.
  • R2 advertises that destination to R1.
  • R2's metric to the destination is 28160.

R1 receives the advertisement and records R2's value as its reported distance. R1 then calculates the complete metric through R2 as 30720.

R1 topology entry: 30720/28160

From R1's perspective:

  • 28160 is R2's reported or advertised distance to 192.168.0.0/24.
  • 30720 is R1's feasible distance through R2.
  • R2 is the successor if no other path has a lower FD.
  • The route through R2 is then normally installed in R1's IP routing table.

Worked EIGRP Metric Interpretation

Router viewing the route: R1
Destination prefix: 192.168.0.0/24
Next-hop neighbor: R2
Displayed FD/RD pair: 30720/28160
Meaning of FD: R1's complete metric to the destination through R2
Meaning of RD: R2's metric to the destination
Route role: Successor when 30720 is the lowest available FD

Successors and Feasible Successors

The successor is the preferred EIGRP next hop. EIGRP normally selects it because it has the lowest feasible distance among the learned paths to a destination.

A feasible successor is a prequalified backup path that satisfies EIGRP's loop-free feasibility condition. The condition is:

candidate neighbor's RD < current successor's FD

The candidate's reported distance must be strictly lower than the successor's feasible distance. This comparison helps EIGRP determine that the neighbor is closer to the destination than the local router's current best total path, supporting loop-free backup selection.

Comparing Two Candidate Paths

Suppose a router learns the same prefix from two neighbors:

  • Neighbor A: FD 30720, RD 28160
  • Neighbor B: FD 35840, RD 25600

Neighbor A is the successor because 30720 is lower than 35840. Neighbor B is a feasible-successor candidate because its RD, 25600, is lower than the successor's FD, 30720.

Two-Path Successor and Feasibility-Condition Comparison

Candidate next hop: Neighbor A
Feasible distance: 30720
Reported distance: 28160
Lowest-FD result: Yes; lowest FD in this example
RD less than successor FD? Not applicable; this is the successor
Potential role: Primary successor

Candidate next hop: Neighbor B
Feasible distance: 35840
Reported distance: 25600
Lowest-FD result: No
RD less than successor FD? Yes; 25600 < 30720
Potential role: Feasible successor backup

A path can be a valid learned route without being a feasible successor. If its RD is not strictly lower than the current successor's FD, EIGRP cannot prequalify it under this condition, even if the path remains in the topology table.

Verifying Values on a Cisco Router

Use the following commands to inspect EIGRP topology information, the installed route, and neighbor relationships:

show ip eigrp topology
show ip eigrp topology 192.168.0.0 255.255.255.0
show ip route 192.168.0.0 255.255.255.0
show ip eigrp neighbors
  • show ip eigrp topology displays learned EIGRP paths and their topology information.
  • show ip eigrp topology 192.168.0.0 255.255.255.0 narrows the inspection to one destination.
  • show ip route 192.168.0.0 255.255.255.0 confirms which successor path was installed in the routing table.
  • show ip eigrp neighbors verifies that the router has formed the expected EIGRP neighbor relationship.

When reading topology output, interpret a pair such as 30720/28160 as feasible distance followed by reported distance. Do not interpret the second number as administrative distance.

Minimal Two-Router Lab Configuration

The following fragments show the structure of a basic lab. Replace the placeholders with the actual transit subnet and wildcard mask used by your interfaces.

router eigrp 100
 network <R1-R2 transit subnet> <wildcard-mask>
 network 192.168.0.0 0.0.0.255
 no auto-summary
  • Use the same EIGRP autonomous-system number, such as 100, on both routers.
  • Apply the destination-LAN network statement only on the router directly connected to 192.168.0.0/24.
  • Use lab-specific interface addresses, transit subnet values, and wildcard masks.
  • Ensure the transit interfaces are active and able to form an EIGRP adjacency.

Troubleshooting FD and RD Problems

The Destination Is Missing from the Topology Table

  • Confirm that an EIGRP neighbor relationship exists.
  • Verify that the destination-connected interface is included in EIGRP.
  • Check interface status and IP reachability between the neighbors.
  • Confirm that a passive-interface setting is not preventing neighbor formation on the transit link.

The Second Number Is Mistaken for Administrative Distance

Recheck the context. In EIGRP topology output, the pair is generally FD/RD: feasible distance first and reported or advertised distance second. The IP routing table commonly uses bracketed administrative-distance/metric notation, which is a different format and represents different concepts.

The Expected Path Is Not the Successor

  • Compare the feasible distances for all learned paths. The lower FD normally wins.
  • Verify interface bandwidth and delay values because they affect the EIGRP composite metric.
  • Check whether unequal-cost load balancing or metric configuration changes are influencing the installed routes.

A Backup Route Is Not a Feasible Successor

  • Compare the candidate path's RD with the current successor's FD.
  • Confirm that the candidate RD is strictly lower than the successor FD.
  • Remember that a learned path may remain in the topology table without meeting the loop-free feasibility condition.

Key Exam Notes

  • RD and AD are alternate EIGRP names for the neighbor's advertised metric.
  • EIGRP advertised distance is not administrative distance.
  • FD is the local router's complete metric through a particular neighbor.
  • In the common notation FD/RD, the first number is FD and the second number is RD.
  • The lowest FD normally selects the successor.
  • A feasible successor must have an RD strictly lower than the current successor's FD.
  • In the example 30720/28160, R1's total metric is 30720 and R2's advertised metric is 28160.

For a focused review of these values and their route-selection role, see EIGRP reported and feasible distance explained.