EIGRP Tables: Neighbor, Topology, and Routing Tables

Learn how EIGRP neighbor, topology, and routing tables work together, including successors, feasible successors, route states, metrics, verification, and troubleshooting.

EIGRP, or Enhanced Interior Gateway Routing Protocol, does not rely on the IP routing table alone. It maintains separate information for discovering adjacent routers, learning multiple paths, and selecting the paths that the router uses to forward packets.

The three principal EIGRP tables are the neighbor table, the topology table, and the IP routing table. Understanding the relationship among them is essential for CCNA troubleshooting.

How EIGRP Tables Work Together

EIGRP uses a staged process:

EIGRP sends hello packets through enabled, non-passive interfaces.
Routers that successfully establish an adjacency are recorded in the neighbor table.
Neighbors exchange route information. Learned prefixes and their path metrics are stored in the topology table.
DUAL, the Diffusing Update Algorithm, evaluates the available paths and identifies the best loop-free path and any qualified backup paths.
The selected successor path is installed in the global IP routing table for forwarding.

Keeping these data structures separate allows EIGRP to retain backup paths and routing information that are not currently being used for forwarding. If the active path fails, EIGRP can often replace it without relearning the route from scratch.

Comparison of EIGRP Tables

Neighbor table: Records directly connected routers with established adjacencies. Entries are created after successful hello-based neighbor formation. It supplies the peers from which routes can be learned. Verify with show ip eigrp neighbors.

Topology table: Stores destination prefixes and paths learned from EIGRP neighbors, including candidate and backup paths. Entries are created when EIGRP receives route information. It is the source used by DUAL for route selection. Verify with show ip eigrp topology.

IP routing table: Contains preferred routes selected for packet forwarding. Entries are installed after route selection and comparison with other routing sources. Verify with show ip route eigrp.

The EIGRP Neighbor Table

The neighbor table is a list of directly connected routers that have formed an operational EIGRP adjacency. An adjacency is the active neighbor relationship that permits the routers to exchange EIGRP routing information.

Requirements for an Adjacency

Two routers normally need all of the following conditions to form an adjacency:

They must have working Layer 2 and IP connectivity on a shared EIGRP-enabled link.
The EIGRP autonomous system number must match on both routers when using classic EIGRP configuration.
EIGRP must be enabled on the connected interfaces.
The interfaces must not be passive. A passive interface can advertise its connected network but does not send EIGRP hello packets.
EIGRP settings that affect adjacency must be compatible, including authentication where configured and metric K values.
Interface addressing, subnet masks, and other relevant protocol settings must permit the routers to communicate correctly.

A neighbor relationship is required before route exchange can occur through that peer. A router may be physically reachable by ping but still be absent from the EIGRP neighbor table if EIGRP settings do not match.

Hello Packets and Hold Time

EIGRP hello packets discover neighbors and maintain existing adjacencies. Each router expects to receive EIGRP traffic from a neighbor within a configured time interval. The hold time is the amount of time a router waits without receiving EIGRP traffic before declaring the neighbor unavailable.

Hold time is not the same as the hello interval. A hello packet refreshes the neighbor's hold timer. If the timer expires because packets stop arriving, the neighbor is removed from the neighbor table and routes learned through that neighbor may need to be recalculated.

Important Neighbor-Table Fields

Neighbor address: The IP address used to identify the adjacent EIGRP router. An unexpected address can indicate an addressing or interface-selection issue.

Local interface: The local interface through which the neighbor is reachable. It helps identify the failed link or the interface participating in EIGRP.

Hold time: Remaining time before the neighbor is declared unavailable if no EIGRP traffic is received. A continuously decreasing value is normal; expiration is a problem.

Uptime: How long the adjacency has remained established. Frequent resets produce short or repeatedly changing uptime values.

SRTT: Smooth round-trip time, an estimate of reliable EIGRP packet timing to the neighbor.

RTO: Retransmission timeout, used when reliable EIGRP packets need to be retransmitted.

Queue count: The number of EIGRP packets waiting in the neighbor's transmission queue. A persistent nonzero or growing queue may indicate congestion or a neighbor that is not acknowledging traffic.

Sequence number: Tracks EIGRP reliable-packet exchanges. It helps the router determine which updates have been acknowledged.

The EIGRP Topology Table

The topology table is EIGRP's database of routes learned from all EIGRP neighbors. It is organized around destination prefixes, and a prefix can have several possible paths. Some paths may be used immediately, while others remain available as backups.

The topology table is not simply a copy of the routing table. It can contain routes that are not currently installed because another path is preferred, because the path is not yet eligible, or because another routing source has won the route-selection process.

Successor, Feasible Distance, and Reported Distance

A successor is the best loop-free next-hop path to a destination. The successor normally has the lowest calculated metric among eligible paths.

Feasible distance (FD) is the lowest calculated EIGRP metric from the local router to a destination. Once a successor is selected, its total metric becomes the current feasible distance.

Reported distance (RD), also called advertised distance, is the metric a neighbor reports for reaching the destination from that neighbor's perspective. The local router adds the cost of reaching the neighbor to calculate its own total path metric.

Feasible Successors and the Feasibility Condition

A feasible successor is a prequalified loop-free backup path. To qualify, the neighbor's reported distance must be less than the current feasible distance:

Reported distance of neighbor < current feasible distance

This is the feasibility condition. It provides a loop-prevention test. If a backup path meets the condition, EIGRP can use it immediately when the successor fails, without first querying other routers for a replacement.

A path that appears attractive because of its total metric is not automatically a feasible successor. The reported-distance comparison must also pass.

Successor: The best loop-free path. Appears in the topology table and is normally installed in the IP routing table. It provides the normal forwarding path.

Feasible successor: A backup path that passes the feasibility condition. Appears in the topology table and can support rapid failover.

Feasible distance: The lowest total EIGRP metric from the local router to the destination. It is used as the reference value for the feasibility condition.

Reported distance: The neighbor's metric to the destination. It is compared with the current feasible distance.

Passive: A stable route state with a known successor. Passive here describes route state, not a passive interface.

Active: A route state in which EIGRP is searching for a replacement path and may be processing queries and replies.

Passive and Active Routes

A passive route has a stable successor and is not undergoing route recomputation. Passive is the normal, healthy state for a converged EIGRP route.

An active route has lost its usable successor and is undergoing route recomputation. EIGRP may send query packets to neighbors asking whether they can reach the destination. Those neighbors return replies after determining their own available paths.

Active status can be normal during a brief convergence event. It becomes a serious concern when a route remains active for too long or becomes stuck in active (SIA). SIA commonly indicates that a query reply was not received in the expected time, often because of an unreachable or overloaded neighbor, unstable connectivity, or an overly broad query scope.

EIGRP Routing-Table Entries

The IP routing table contains the routes selected for forwarding traffic. EIGRP submits its best eligible routes to the routing table, where they compete with routes from connected, static, and other dynamic sources.

For classic Cisco IOS output, an internal EIGRP route is identified by D. An external EIGRP route is identified by D EX. External routes were redistributed into EIGRP from another routing source or routing process.

Internal EIGRP: Route code D; default administrative distance 90; originated within the same EIGRP autonomous system.

External EIGRP: Route code D EX; default administrative distance 170; redistributed into EIGRP from another source.

Administrative distance is a preference value used when different routing sources offer the same destination. A lower value is preferred. It is separate from the EIGRP metric, which selects among EIGRP paths.

Path Installation and Load Balancing

The successor is normally installed in the routing table. A feasible successor is generally retained only in the topology table as a ready backup; it is not automatically installed as a separate unequal-cost route.

When multiple EIGRP paths have equal metrics, EIGRP can install multiple paths for equal-cost load balancing, subject to the configured maximum-paths value. On many Cisco IOS configurations, the default maximum is four paths, but the platform and configuration should be verified rather than assumed.

router eigrp <autonomous-system>
 maximum-paths <number>

Unequal-cost load balancing uses the variance setting. A feasible successor whose metric falls within the variance range may be installed along with the successor:

router eigrp <autonomous-system>
 variance <multiplier>

Variance does not bypass loop prevention. A path must still satisfy the feasibility condition before it can be considered for unequal-cost installation.

EIGRP Metric and Route Selection

EIGRP uses a composite metric rather than hop count alone. With the default K values, the principal metric components are bandwidth and delay.

Bandwidth represents the slowest bandwidth along the path and is used as a capacity-related component.
Delay represents the cumulative configured delay of the interfaces along the path.
Reliability and load are available metric inputs, but they are not normally used when the default K values are configured.

Neighboring routers must use compatible K values. A K-value mismatch can prevent adjacency formation because the routers might calculate metrics differently. Metric consistency also depends on accurate interface bandwidth and delay values.

Among eligible EIGRP paths, the path with the lowest feasible distance is selected as the successor. The metric is not the same as administrative distance: metric compares paths within EIGRP, while administrative distance compares EIGRP with other route sources.

Basic EIGRP Lab Configuration

The following classic EIGRP configuration illustrates the relationship between an autonomous system number and participating interfaces:

router eigrp <autonomous-system>
 network <network-address> <wildcard-mask>
 no auto-summary

The autonomous system number must match on intended neighbors. Network statements determine which interfaces participate in the EIGRP process. Use a passive interface on a user-facing network when the network should be advertised but no EIGRP neighbor should form there.

router eigrp <autonomous-system>
 passive-interface <interface>
 no passive-interface <interface>

EIGRP Verification Workflow

Check EIGRP from the physical and lower layers upward. This prevents spending time interpreting topology output when the underlying interface is down.

Verify interfaces and addressing. Use show ip interface brief, show interfaces <interface>, and ping <neighbor-address>.
Verify protocol participation. Use show ip protocols, show running-config | section router eigrp, and show ip eigrp interfaces detail.
Verify neighbors. Use show ip eigrp neighbors and, when needed, show ip eigrp neighbors detail.
Inspect learned paths. Use show ip eigrp topology, show ip eigrp topology <destination-prefix>, or show ip eigrp topology all-links.
Confirm installation. Use show ip route eigrp and show ip route <destination-prefix>.

Reading the Output

An expected router absent from show ip eigrp neighbors means the adjacency problem must be solved before routes can be learned through that peer.
A prefix in the topology table but not in the routing table may have lost to another route source, may not have an eligible path, may be filtered, or may still be converging.
A topology entry marked passive is normally stable. An active entry indicates route recomputation or query processing.
Topology output showing a successor and another path with a qualifying reported distance indicates that rapid failover may be possible.
A routing-table entry beginning with D is internal EIGRP; D EX identifies an external EIGRP route.

Failure and Convergence Behavior

Failure with a Feasible Successor

When a neighbor disappears, EIGRP removes the adjacency after the hold timer expires or after another failure is detected. If the affected destination has a feasible successor, DUAL can promote that backup path to successor and install it in the routing table. This local repair is usually rapid because the path has already passed the loop-free feasibility test.

Failure without a Feasible Successor

If no feasible successor exists, EIGRP cannot safely choose an unverified path immediately. The route can transition from passive to active. The router sends queries to appropriate neighbors, and those neighbors return replies describing whether they can find a route. After the required replies arrive, DUAL selects a replacement path or removes the destination.

A short active period can be expected during convergence. A route that remains active because replies are delayed or missing may become stuck in active. Investigate neighbor reachability, interface errors, overloaded routers, unstable links, and route summarization or topology design that could limit query scope.

Practical Scenarios

Two-Router Adjacency Verification

Configure EIGRP on the shared subnet of two routers using the same autonomous system number. Confirm that each router lists the other with show ip eigrp neighbors. Check the neighbor address, local interface, uptime, hold timer, and queue count. A stable uptime and an empty queue are generally reassuring; repeated uptime resets or a growing queue deserve investigation.

Single Successor Route

In a three-router lab, make one remote LAN reachable through one neighbor. Use show ip eigrp topology <destination-prefix> to inspect the route, feasible distance, and successor. Then use show ip route <destination-prefix> to confirm the corresponding D route and next hop in the IP routing table.

Successor and Feasible Successor

With two unequal paths to the same destination, compare each neighbor's reported distance with the current feasible distance. If the backup neighbor's reported distance is lower, it qualifies as a feasible successor. After the primary link fails, observe the backup path becoming the successor without the same query process required when no feasible successor exists.

No Feasible Successor

In a controlled lab, fail the primary path where the remaining path does not meet the feasibility condition. Inspect the destination with show ip eigrp topology <destination-prefix>. The route may become active while queries and replies are processed, then either be replaced or removed.

Internal and External Routes

Redistribute a test route into EIGRP and compare it with an internally originated route. The internal route appears as D with a default administrative distance of 90. The redistributed route appears as D EX with a default administrative distance of 170.

Equal-Cost and Unequal-Cost Paths

Two equal-metric paths can be installed when the configured maximum-paths permits them. To test unequal-cost behavior, first identify a feasible successor, then apply a controlled variance value. Emphasize that variance can make an eligible path installable; it cannot make a path that fails the feasibility condition safe for use.

Troubleshooting by Symptom

Expected neighbor is absent: Inspect the neighbor table and interface/protocol configuration first. Common causes include a down interface, incorrect IP addressing, mismatched autonomous system numbers or K values, a passive interface, authentication mismatch, or missing Layer 2 connectivity. Use show ip interface brief, show ip protocols, show ip eigrp interfaces detail, and ping the directly connected neighbor.

Neighbor exists but route is missing: Inspect the topology entry. The route may not have been advertised, may be filtered, may be absent from the neighbor's own topology, may have lost to a better administrative distance, or may be converging. Use show ip eigrp topology <destination-prefix>, show ip route <destination-prefix>, and the EIGRP configuration.

Route is active: Inspect the destination's topology state and neighbor details. Possible causes include successor failure, no feasible successor, pending query replies, or an unreachable neighbor.

Route is stuck in active or convergence is slow: Check all relevant neighbors, interface errors, unstable links, overloaded devices, and route-summary boundaries. A large query scope can make recovery slower.

Backup path does not take over: Compare the backup path's reported distance with the current feasible distance. The path may fail the feasibility condition, depend on a failed neighbor, or be confused with unequal-cost installation behavior.

Exam-Relevant Notes

The neighbor table contains adjacent EIGRP routers, not every router in the EIGRP domain.
The topology table can contain multiple paths for one destination; the routing table normally contains only the selected forwarding paths.
A successor is the best path. A feasible successor is a prequalified backup path.
The feasibility condition is reported distance less than feasible distance.
Passive route state is different from a passive interface.
Internal EIGRP uses route code D and default administrative distance 90.
External EIGRP uses route code D EX and default administrative distance 170.
EIGRP's default metric uses bandwidth and delay. Reliability and load are not normally used with default K values.
A feasible successor enables rapid local convergence. Without one, EIGRP may place the route into active query processing.

For related routing foundations, review the Computer Network Explained lesson and compare EIGRP's neighbor and topology information with the concepts introduced in Configure OSPF.