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EIGRP Tables: Neighbor, Topology, and Routing Tables

Learn how EIGRP neighbor, topology, and routing tables work together, and verify each one with Cisco IOS show commands.

EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol. It maintains separate information for its peer relationships, learned route candidates, and routes selected for packet forwarding.

Understanding these three data structures makes EIGRP troubleshooting much easier. A route can exist in the topology table without appearing in the IP routing table, and a router can have a neighbor relationship without learning a particular destination through that neighbor.

The Three EIGRP Tables

EIGRP uses three related but different tables:

• Neighbor table: Records established EIGRP adjacencies with directly connected peer routers.
• Topology table: Stores route information learned from neighbors, including paths that are not currently used for forwarding.
• IP routing table: Contains the routes selected by the router for packet-forwarding decisions.

These tables do not contain identical information. The neighbor table lists routers, not destination networks. The topology table can contain multiple paths to one prefix. The routing table normally contains only the best eligible path, although multiple equal-cost paths may be installed.

EIGRP Neighbor Table

An EIGRP neighbor adjacency is a functioning peer relationship between two directly connected routers. Before routers can exchange routing information, they must discover one another and form this relationship.

Neighbor formation relies on EIGRP Hello packets. A router sends Hellos through an interface participating in EIGRP. A neighboring router must receive compatible Hellos on a common eligible network segment. Configuration compatibility, interface operation, and relevant EIGRP parameters must also allow the adjacency to form.

The neighbor table is therefore a record of directly connected EIGRP peers. It is not a list of remote destination prefixes. For example, if R1 forms adjacencies with routers at 10.0.0.2 and 192.168.0.2, those two peer addresses appear in R1's neighbor table even though the table does not describe every network reachable through them.

Common Neighbor Table Fields

The hold timer is refreshed when valid EIGRP communication is received. If the timer expires, EIGRP assumes that the peer is no longer reachable or operational. The neighbor entry is removed, and routes learned through that neighbor are reevaluated.

EIGRP uses reliable delivery for certain packet exchanges. SRTT provides a smoothed estimate of how long communication takes, and RTO uses timing information to determine when a reliable packet should be retransmitted. These values are not route metrics; they describe the health and timing of the neighbor relationship.

Verifying Neighbors

R1# show ip eigrp neighbors

Use this command to display established IPv4 EIGRP neighbors and adjacency timing information. On IOS versions that support it, the following command provides expanded details:

R1# show ip eigrp neighbors detail

When an expected peer is missing, first verify the interface state, Layer 3 addressing, EIGRP participation, matching autonomous system or process configuration, and other essential compatibility requirements.

EIGRP Topology Table

The topology table is EIGRP's collection of learned route candidates. It is broader than the IP routing table because it retains paths that are not currently selected for forwarding.

Topology information is maintained for a particular EIGRP autonomous system or EIGRP process and address family. A topology entry commonly includes:

• The destination prefix and prefix length.
• The advertising next hop or neighboring router.
• The outgoing local interface.
• The route origin, such as the neighbor that advertised the prefix.
• The calculated composite metric.
• The feasible distance (FD), which is the local router's total calculated metric to the destination.
• The reported distance (RD), also called advertised distance, which is the metric to the destination as reported by a neighbor.

A destination can have several paths in the topology table. EIGRP evaluates those paths and identifies a successor, the best loop-free path selected for installation.

Successors and Feasible Successors

A feasible successor is a prequalified loop-free backup path. It must satisfy EIGRP's feasibility condition: the neighbor's reported distance must be lower than the current feasible distance.

A feasible successor can be promoted quickly if the successor fails because EIGRP already knows that the backup path is loop-free. Not every alternate topology entry is a feasible successor. An alternate path may remain in the topology table but fail the feasibility condition, so it cannot be used as an immediate backup.

Some destinations have no feasible successor. If their active successor fails, EIGRP must search for another route rather than immediately promoting a prequalified backup.

Passive and Active Routes

A passive route is stable. EIGRP is not currently recomputing that destination.

An active route is undergoing route recomputation. EIGRP is querying neighbors for an alternative path after it no longer has a usable successor. A prolonged active state can indicate query propagation, missing replies, or a topology problem.

Verifying the Topology Table

R1# show ip eigrp topology

This command displays topology entries, candidate paths, metrics, successor information, and route state. To inspect one prefix in detail, use:

R1# show ip eigrp topology 172.16.0.0 255.255.0.0

When reading the output, distinguish the selected successor from other learned paths. The next-hop address and outgoing interface shown for a candidate should identify the neighbor and link through which that candidate was learned.

IP Routing Table

The IP routing table is the forwarding table used during route lookup. When a packet arrives, the router compares its destination address with routing-table entries and chooses the appropriate forwarding path.

EIGRP does not install every topology-table path into the routing table. It installs the best eligible EIGRP route, normally the successor, provided that EIGRP's route is selected over competing routing sources according to route-selection rules such as administrative distance and metric.

Alternate paths can remain visible in the topology table while only one path appears in the routing table. Multiple EIGRP paths can be installed when they meet the requirements for equal-cost multipath. EIGRP can also support unequal-cost load balancing with the variance setting, which permits eligible paths within a configured metric range. That is different from the normal single-best-path example.

Verifying Installed EIGRP Routes

R1# show ip route eigrp

This command displays EIGRP-derived routes currently installed in the IPv4 routing table. To inspect the selected route for one network, use:

R1# show ip route 172.16.0.0

Correlate the routing-table next hop with the neighbor address in show ip eigrp neighbors. Then correlate the routing-table outgoing interface with the local interface shown for that neighbor and with the interface listed for the topology-table path.

How Route Information Flows

1. Neighbors form: Routers exchange compatible Hello packets and establish an adjacency.
2. Routes are received: The neighbor advertises reachable prefixes and associated metric information.
3. Routes enter the topology table: EIGRP records and evaluates each candidate path, including alternatives.
4. A successor is selected: EIGRP chooses the best loop-free path. A qualifying backup may be recorded as a feasible successor.
5. The route is installed: The selected eligible route is placed in the IP routing table for forwarding.

If a neighbor is lost, its neighbor-table entry is removed after the hold timer expires or another failure is detected. Paths learned through that neighbor are then invalidated or reevaluated in the topology table. If a feasible successor exists, EIGRP can promote it. Otherwise, the route may become active while EIGRP queries neighbors for a replacement.

Three-Router Interpretation Exercise

Assume R1 has two EIGRP neighbors:

• Neighbor 10.0.0.2 is reached through one local interface.
• Neighbor 192.168.0.2 is reached through a different local interface.
• Both neighbors advertise reachability to 172.16.0.0/16.
• The path through 10.0.0.2 has the lower feasible distance.

Representative verification sequence:

R1# show ip eigrp neighbors
R1# show ip eigrp topology 172.16.0.0 255.255.0.0
R1# show ip route eigrp
R1# show ip route 172.16.0.0

The first command should identify the two peer addresses and their local interfaces. The second should show two candidate paths to the remote prefix and identify the preferred successor. The third and fourth should show the installed route, normally using 10.0.0.2 as the next hop because it has the best metric.

The route counts differ because the neighbor table counts peer relationships, the topology table counts learned route candidates, and the routing table counts selected forwarding routes. Two neighbors do not necessarily mean two routes to every destination, and two topology paths do not necessarily mean two installed forwarding paths.

Troubleshooting by Comparing the Tables

Expected Neighbor Is Missing

Run show ip eigrp neighbors. Check interface status, addressing, Layer 3 reachability, EIGRP-enabled interfaces, matching autonomous system or process configuration, and essential EIGRP compatibility parameters.

Without a neighbor adjacency, R1 cannot learn routes from that peer. Consequently, paths through that peer will not appear in the topology table or the routing table.

Prefix Is in the Topology Table but Not the Routing Table

Inspect the prefix with show ip eigrp topology, then compare it with show ip route. The EIGRP path may not be the selected best route, may be superseded by a route from another source with a lower administrative distance, or may otherwise be ineligible for installation.

This is normal evidence that topology information is broader than installed forwarding information.

No Immediate Backup Exists

Inspect the topology entry for a feasible successor. If alternate paths fail the feasibility condition, they are not immediate backups even though they remain known to EIGRP. When the successor fails, the destination may enter the active state while EIGRP queries neighbors.

Neighbor Repeatedly Disappears

Use show ip eigrp neighbors and observe hold time, uptime, SRTT, and RTO. Check for unstable links, packet loss, interface errors, and inconsistent EIGRP settings. Confirm which local interface corresponds to the neighbor entry.

Adjacency instability repeatedly removes and relearns the routes associated with that neighbor, which can cause changes in both the topology and routing tables.

Additional Verification

Use the following command to verify the EIGRP process and participating interfaces during basic troubleshooting:

R1# show ip protocols

This output can help confirm the EIGRP process information, participating interfaces, and networks being advertised. Use it together with the three table-specific commands rather than treating it as a replacement for them.

Exam-Ready Summary

• The neighbor table lists established directly connected EIGRP peers.
• Hello packets help discover and maintain neighbor adjacencies.
• The hold timer reaching zero causes a neighbor to be declared unavailable and removed.
• SRTT and RTO support reliable EIGRP packet delivery; they are not route metrics.
• The topology table contains learned route candidates, including alternatives not installed for forwarding.
• FD is the local total metric; RD is the metric reported by a neighbor.
• The successor is the best loop-free path selected for installation.
• A feasible successor is a loop-free backup that satisfies the feasibility condition.
• A passive route is stable; an active route is being recomputed through queries.
• The routing table contains routes selected for packet forwarding, not every topology-table path.
• Use show ip eigrp neighbors for adjacencies, show ip eigrp topology for EIGRP candidates, and show ip route eigrp or show ip route for installed routes.

For related study, see EIGRP Tables.