EIGRP Reported Distance and Feasible Distance Explained
Learn how EIGRP calculates reported distance and feasible distance, selects successors, keeps loop-free backup paths, and handles route failures.
EIGRP, or Enhanced Interior Gateway Routing Protocol, is an advanced distance-vector routing protocol that uses the DUAL (Diffusing Update Algorithm) to select loop-free routes. Two values are central to that process: reported distance (RD) and feasible distance (FD).
RD describes the distance a neighbor reports to a destination. FD describes the complete distance calculated by the local router through that neighbor. Understanding the difference makes EIGRP topology-table output, successor selection, and backup-route behavior much easier to interpret.
EIGRP Metric Fundamentals
EIGRP does not select routes by simple hop count. It uses a composite metric calculated from interface characteristics and EIGRP metric weights, called K values.
With the default K values, EIGRP uses:
- Bandwidth: the minimum bandwidth along the path.
- Delay: the sum of the delays of the interfaces along the path.
EIGRP can also include reliability and load when the K values are configured to use them. MTU is carried with route information, but it does not influence the default composite metric calculation.
Metric component: Bandwidth
Used by default: Yes
Impact: The lowest bandwidth on the path contributes to the metric.
Operational consideration: The IOS bandwidth value is a routing metric input, not necessarily the physical link speed.
Metric component: Delay
Used by default: Yes
Impact: Interface delays are accumulated across the path.
Operational consideration: Changing delay can change route selection.
Metric component: Reliability
Used by default: No
Impact: Can influence the metric when its K value is enabled.
Operational consideration: Dynamic values can make route behavior less predictable.
Metric component: Load
Used by default: No
Impact: Can influence the metric when its K value is enabled.
Operational consideration: A changing load value can cause metric changes.
Metric component: MTU
Used by default: No
Impact: It is carried in route information but is not part of the default metric calculation.
Operational consideration: MTU still matters for packet forwarding and should be checked separately.
The default EIGRP metric is commonly expressed conceptually as:
Metric = 256 × (K1 × scaled bandwidth + K2 × scaled bandwidth / (256 − load) + K3 × scaled delay) × K5 / (reliability + K4)In the default configuration, K1 and K3 are 1, while K2, K4, and K5 are 0. Therefore, the practical default calculation uses bandwidth and delay. Interface bandwidth and delay settings affect the calculated path metric, which in turn affects FD and route selection.
All EIGRP neighbors must use matching K values. If their K values do not match, they will not form an EIGRP adjacency.
Reported Distance and Advertised Distance
Reported distance (RD) is the metric that a neighboring router reports for its own path to a destination. It is also called advertised distance (AD). These are two names for the same EIGRP value.
RD represents only the neighbor-to-destination portion of the path. It does not include the metric contribution from the local router to that neighbor.
For example, suppose Router A receives a route advertisement from Router B. Router B reports an RD of 20,000 for 192.0.2.0/24. Router A uses that value as the downstream portion of its own path calculation.
Feasible Distance
Feasible distance (FD) is the local router's complete calculated metric to reach a destination through a particular next-hop neighbor.
Conceptually:
Local path metric = local link metric contribution + neighbor's RDIf Router A's metric contribution to Router B is 10,000 and Router B reports an RD of 20,000, Router A calculates:
FD through Router B = 10,000 + 20,000 = 30,000FD is stored for each destination in the EIGRP topology table. The path with the lowest feasible distance normally becomes the successor, and the successor is installed in the IP routing table.
FD and RD: The Important Difference
FD is the local router's total distance. RD is the neighbor's claimed distance. They describe different portions of the same path.
- FD: local router to neighbor, plus neighbor to destination.
- RD: neighbor to destination only.
- FD is local: different routers can calculate different FDs for the same destination.
- RD is received from a neighbor: it describes the neighbor's route calculation.
Routes learned through different neighbors can have different FDs and RDs for the same prefix because each path has different link metrics and downstream information.
A route can have a relatively low FD and still fail the feasibility condition. The feasibility condition does not compare a candidate's FD with the successor's FD. It compares the candidate's RD with the successor's FD.
Successor Route Selection
A successor is EIGRP's best next-hop path to a destination. Under normal selection, the path with the lowest FD becomes the successor.
The successor appears in both places:
- The EIGRP topology table records the destination and successor path.
- The IP routing table uses the successor for forwarding packets.
If multiple paths have equal eligible metrics, EIGRP can use equal-cost paths. EIGRP can also support conceptual unequal-cost load balancing. The variance command permits eligible paths whose metrics are higher than the best path, subject to EIGRP's feasibility rules. The maximum-paths command limits the number of installed paths.
router eigrp 100
variance 2
maximum-paths 4Feasibility Condition and Feasible Successors
The feasibility condition is:
Candidate neighbor's RD < current successor's FDA neighbor that satisfies this condition qualifies as a feasible successor. DUAL uses this rule as a loop-free backup-path test. The reasoning is that the neighbor claims a distance smaller than the distance the local router already uses through its successor, so the neighbor is not relying on the local router's route to reach the destination.
A feasible successor may have a higher total FD than the successor. For example:
- Neighbor A: FD 30,000 and RD 20,000. It becomes the successor.
- Neighbor B: FD 35,000 and RD 25,000. Its FD is higher, but its RD is less than 30,000, so it is a feasible successor.
If Neighbor B instead reports RD 31,000, it fails because 31,000 is not less than the successor FD of 30,000. The path may remain in the topology table as an alternate path, but it is not an immediate feasible successor.
Successor, Feasible Successor, and Non-Feasible Path
- Successor: the best eligible path, normally the path with the lowest FD.
- Feasible successor: a backup path whose RD is lower than the current successor's FD.
- Non-feasible path: a learned path that does not satisfy the feasibility condition. It cannot be used as an immediate loop-free backup.
EIGRP Topology Table Interpretation
The topology table is EIGRP's database of learned destination networks and the paths reported by each neighbor. A destination entry can contain several per-neighbor path entries.
A representative entry might look like this:
P 192.0.2.0/24, 1 successors, FD is 30000
via 10.0.0.2 (30000/20000), GigabitEthernet0/0
via 10.0.1.2 (35000/25000), GigabitEthernet0/1In the metric pair shown as (FD/RD):
(30000/20000)means FD 30,000 and RD 20,000.(35000/25000)means FD 35,000 and RD 25,000.- The first path is the successor because it has the lower FD.
- The second path is a feasible successor because RD 25,000 is lower than the successor FD of 30,000.
When interpreting topology output, identify the destination, successor count, FD, next hop, outgoing interface, FD/RD pair, and route state. A passive route is stable and has completed route computation. An active route is undergoing recomputation and may be waiting for replies to EIGRP queries.
The topology table can contain more paths than the IP routing table. EIGRP selects eligible paths for installation; unselected paths remain available for evaluation or backup according to their status.
Route Loss, Queries, and Active State
When a successor fails and a feasible successor is already present, DUAL can promote that backup immediately. The router does not need to send EIGRP queries for that destination.
When no feasible successor exists, the router cannot safely assume that an alternate path is loop-free. It sends queries to neighbors and waits for replies. During this process, the destination can enter active state.
Stuck in active (SIA) occurs when required EIGRP query replies are not received within the expected process. SIA can result from unstable neighbors, overloaded routers, filtering, broken return paths, or overly large query domains. The feasibility condition reduces query activity by allowing safe backups to be used locally.
Metric Changes and K Values
Changing an interface's bandwidth or delay changes the local metric contribution. That can change a path's FD, alter the successor, or cause a path to gain or lose feasible-successor eligibility.
interface GigabitEthernet0/0
delay 1000
bandwidth 100000Use such changes only in a controlled lab or after planning their effects. A bandwidth value influences routing calculations and can affect protocols beyond EIGRP.
Classic IPv4 EIGRP configuration can be started with:
router eigrp 100
network 10.0.0.0 0.0.0.255
network 192.0.2.0 0.0.0.3
no auto-summaryThe autonomous system number must match between intended neighbors. K values must also match. Passive interfaces, mismatched subnets, filtering, and authentication inconsistencies can prevent adjacency formation and therefore prevent FD/RD information from being learned.
Verification Workflow
Follow this order when investigating EIGRP route selection:
- Verify that the expected neighbor adjacency exists.
- Inspect all paths for the destination in the topology table.
- Compare each path's FD/RD pair.
- Apply the feasibility condition using the current successor FD.
- Confirm the installed successor in the IP routing table.
- Check protocol settings and interface metric inputs if values are unexpected.
Troubleshooting Common Problems
Expected backup is not a feasible successor
Compare the candidate's RD with the current successor's FD using show ip eigrp topology. The candidate must have an RD strictly lower than the successor FD. Also verify that the path was learned through an active neighbor and that filtering, summarization, or an incorrect network statement is not preventing the advertisement.
A route becomes active after the successor fails
This normally means no feasible successor was available. Inspect the destination in the topology table, check alternate FD/RD values, verify neighbor stability, and look for EIGRP query or reply activity.
The selected neighbor is not the one expected
The installed path may simply have the lower FD. Check interface bandwidth and delay, metric weights, route summarization, and every candidate path rather than relying on assumed link speeds.
Neighbors do not form
Check the autonomous system number, K values, IP addressing, interface state, passive-interface settings, authentication, access-control filters, and whether hello packets can reach the neighbor.
Metric values change unexpectedly
Inspect interface bandwidth and delay with show interfaces and the running configuration. Compare topology-table FD/RD values before and after the change, and check whether only one part of the topology was modified.
Exam-Relevant Summary
- RD and AD are equivalent terms.
- RD is the neighbor's distance to the destination, not the complete local path.
- FD is the local router's total metric through a particular neighbor.
- FD is calculated from the local link contribution plus the neighbor's RD.
- The lowest eligible FD normally becomes the successor.
- A feasible successor satisfies
RD < successor FD. - A feasible successor can have a higher FD than the successor.
- A feasible successor provides a precomputed loop-free backup and can be promoted without queries.
- Without a feasible successor, the destination may become active and EIGRP sends queries.
- Unanswered queries can cause a stuck-in-active condition.
- Default EIGRP metric calculation uses bandwidth and delay; reliability and load are optional, and MTU does not affect the default composite metric.
- Matching EIGRP K values are required for neighbor adjacency.
For broader routing context, review the computer network explained lesson and compare EIGRP route behavior with OSPF configuration and route selection.