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EIGRP Configuration and Verification for Cisco Routers

Learn how EIGRP works and how to configure, verify, troubleshoot, summarize, and control IPv4 routes on Cisco IOS routers.

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-developed advanced distance-vector interior gateway protocol. It dynamically exchanges IPv4 routes between routers that belong to the same EIGRP autonomous system (AS).

This lesson focuses on classic IPv4 EIGRP configuration on Cisco IOS. It covers the protocol fundamentals, a two-router lab, verification commands, route summarization, administrative distance, and common troubleshooting techniques.

Prerequisites

• IPv4 addressing, subnet masks, and CIDR notation
• Wildcard mask calculation
• Basic Cisco IOS configuration modes
• Router interface configuration and status verification
• Connected and static routes
• Routing-table interpretation
• Ping and traceroute

Review the OSI reference model and computer networking fundamentals if these prerequisites are unfamiliar.

EIGRP Overview and Purpose

EIGRP is an interior routing protocol, meaning it is intended for routing within one organization or routing domain. In classic EIGRP, routers use an autonomous system number to identify the EIGRP process. Routers that should become neighbors must use the same AS number on their shared link.

EIGRP is commonly used when an enterprise needs dynamic route exchange, relatively fast convergence, support for route summarization, and flexible metric-based path selection. It is not the same as an external BGP autonomous system; the EIGRP AS number identifies the EIGRP routing process within the local network.

Classic mode and named mode

Classic EIGRP uses commands such as router eigrp 100 followed by network statements. Named-mode EIGRP uses a named process and separates address-family configuration from other EIGRP settings. Named mode supports additional structure and features, but classic IPv4 EIGRP remains useful for foundational IOS labs and exam practice.

How EIGRP Operates

Neighbor discovery

EIGRP routers send Hello packets on EIGRP-enabled interfaces. A router can form an adjacency only with a directly connected router whose interface can exchange EIGRP packets successfully.

For an adjacency to form, the routers generally need compatible interface addressing, the same classic EIGRP AS number, matching EIGRP-relevant parameters, and working Layer 1 and Layer 2 connectivity. EIGRP Hellos are sent to a multicast address rather than to ordinary end hosts.

Reliable updates and partial updates

EIGRP uses a reliable transport mechanism for updates that require delivery confirmation. The receiving router acknowledges reliable update packets. If an update is lost, EIGRP can retransmit it.

After an adjacency is established, EIGRP does not continuously send the entire routing table whenever a small change occurs. It sends bounded, or partial, updates to affected neighbors. This reduces unnecessary traffic and allows changes to converge efficiently.

EIGRP tables

Each EIGRP router maintains several kinds of information:

• Neighbor table: Directly connected EIGRP routers and information such as their hold timers and interface.
• Topology table: Routes learned from neighbors, their metrics, and candidate paths.
• Routing table: The best routes selected for forwarding traffic.

DUAL evaluates the topology information and selects loop-free paths. It identifies a best path, called the successor, and may retain a qualified backup path, called a feasible successor.

EIGRP Metric Concepts

EIGRP uses a composite metric. By default, the metric uses bandwidth and delay. The minimum bandwidth along the complete path and the cumulative delay across the path influence the result.

Load and reliability are possible EIGRP metric inputs, but the default K values do not use them. Changing K values changes the metric formula and requires matching values between neighbors.

Feasible distance (FD) is the local router's calculated total metric to a destination. Reported distance (RD), also called advertised distance, is the metric a neighbor reports for reaching that destination.

The successor is the best loop-free next hop selected for the routing table. A feasible successor is a prequalified loop-free backup path. DUAL uses the feasibility condition to prevent a backup route from creating a loop.

Prerequisites for a Working Deployment

1. Assign valid IPv4 addresses and masks to all routed interfaces.
2. Confirm that interfaces are enabled with no shutdown.
3. Confirm Layer 1 and Layer 2 connectivity, including cabling, VLANs, and encapsulation where applicable.
4. Use the same classic EIGRP AS number on routers intended to become neighbors.
5. Ensure EIGRP-relevant parameters match, including K values and authentication settings when authentication is configured.
6. Make sure the EIGRP network statements match the intended local interfaces.

Two-Router EIGRP Lab

Use this topology:

• R1 and R2 share transit network 10.12.12.0/30.
• R1 uses LAN 192.168.10.0/24.
• R2 uses LAN 192.168.20.0/24.
• Both routers use classic EIGRP AS 100.

For example, R1 may use 10.12.12.1/30 and R2 may use 10.12.12.2/30 on their shared interfaces.

R1 configuration

configure terminal
router eigrp 100
 network 10.12.12.0 0.0.0.3
 network 192.168.10.0 0.0.0.255
 no auto-summary
 passive-interface GigabitEthernet0/0
end

R2 configuration

configure terminal
router eigrp 100
 network 10.12.12.0 0.0.0.3
 network 192.168.20.0 0.0.0.255
 no auto-summary
 passive-interface GigabitEthernet0/0
end

Replace the interface names and addresses with those in your topology. The LAN-facing interface is passive in this example. The router-to-router interface must not be passive, because it needs to send and receive EIGRP Hellos.

Understanding network statements

A classic EIGRP network statement enables EIGRP on local interfaces whose addresses match the statement. The connected networks on those interfaces are then eligible for advertisement to EIGRP neighbors.

The second value is a wildcard mask. A wildcard mask is an inverse mask: zero bits must match, while one bits can vary.

network 192.168.10.0 0.0.0.255

This matches addresses in the entire 192.168.10.0/24 range. A major-network-style statement can be broader:

network 192.168.0.0 0.0.255.255

For precision, match an individual interface address or subnet. For example, if an interface has address 10.12.12.1/30, an exact host match is:

network 10.12.12.1 0.0.0.0

Exact or narrowly scoped statements reduce the chance of unintentionally enabling EIGRP on an interface. The command does not directly place an arbitrary network into EIGRP; it matches local interfaces. The interface's connected network is what EIGRP advertises.

Disabling legacy auto-summary

no auto-summary disables legacy classful summarization behavior in classic IOS lessons. This is important in discontiguous or variable-length subnet mask designs, where automatic classful boundaries could produce misleading advertisements. IOS versions and EIGRP modes differ, so verify the behavior on the platform being used.

Passive interfaces

A passive interface continues to advertise its connected network through EIGRP, but it does not send EIGRP Hellos on that interface. This is appropriate for a user LAN where no EIGRP router is expected.

configure terminal
router eigrp 100
 passive-interface default
 no passive-interface GigabitEthernet0/1
end

This safer pattern makes every interface passive first, then explicitly permits Hellos on the router-facing interface. Repeat no passive-interface for every interface that must form an adjacency.

Route Advertisement and Route Control

Connected route advertisement

When an interface is matched by an EIGRP network statement and is operational, its connected network can be advertised to EIGRP neighbors. A neighbor installs that prefix only if it receives the advertisement and selects it as a valid route.

If the interface is passive, the network can still be advertised. Passive status suppresses neighbor discovery on that interface; it does not suppress the connected prefix from EIGRP advertisements.

Manual route summarization

Manual summarization combines several more-specific prefixes into one aggregate prefix. A summary can reduce routing-table size, limit update scope, and hide internal topology from an upstream router.

configure terminal
interface GigabitEthernet0/1
 ip summary-address eigrp 100 172.16.8.0 255.255.248.0
end

This command advertises 172.16.8.0/21 out the selected interface. A summary route is also locally generated on the router that creates the summary, allowing the router to represent the aggregate even while individual component routes change. Verify that the aggregate covers only networks that should be reachable. A 172.16.8.0/21 summary covers addresses from 172.16.8.0 through 172.16.15.255; it is broader than only 172.16.10.0/24 through 172.16.13.0/24.

Summaries must be applied in the correct outbound direction. A downstream router may receive only the aggregate rather than the individual routes, so test reachability to every intended component network before deployment.

Redistribution caution

Redistribution imports routes from another routing source, such as static routing or OSPF, into EIGRP. An imported route is an external EIGRP route. EIGRP needs a seed metric for redistributed routes, commonly supplied with a redistribution metric or a default metric.

Redistribution can create routing loops, suboptimal paths, and unexpected route feedback. Treat it as an advanced design task and use explicit filtering and documented metrics.

Verification Workflow

Use a bottom-up workflow. Do not begin with EIGRP commands if the interface itself is down.

1. Check interface addressing and status.
2. Check which interfaces EIGRP has enabled.
3. Check neighbor adjacencies.
4. Inspect the EIGRP topology table.
5. Inspect installed EIGRP routes.
6. Confirm the selected path, next hop, administrative distance, and metric.
7. Test forwarding with ping and traceroute.

Reading a route

show ip route eigrp
D    192.168.20.0/24 [90/307200] via 10.12.12.2, 00:00:18, GigabitEthernet0/1

The code D identifies an internal EIGRP route. The first number in brackets, 90, is the administrative distance. The second, 307200, is the EIGRP metric. The next-hop address is 10.12.12.2, and the final value identifies the outgoing interface.

Administrative Distance and EIGRP Path Selection

Administrative distance (AD) ranks the trustworthiness of different routing sources. A router first compares AD when routes to the same destination come from different protocols or sources. After EIGRP wins that comparison, EIGRP uses its composite metric to select among EIGRP paths.

Equal-cost paths can be installed for load balancing when their EIGRP metrics meet the platform's equal-cost criteria. The variance setting can permit unequal-cost load balancing over eligible feasible successor routes. Variance does not make every alternative path usable; the path must still satisfy EIGRP's loop-free requirements.

Troubleshooting EIGRP Adjacencies

If the neighbor table is empty, check physical and IP conditions before analyzing route metrics. In the failed-adjacency example where R1 uses AS 100 and R2 uses AS 200, the directly connected /30 can be correct while the neighbor relationship remains absent. Correct R2 to AS 100, then verify that a neighbor entry appears.

Troubleshooting Missing Routes

An adjacency can be healthy while a particular remote LAN is absent. Check whether the advertising router's LAN interface is operational and matched by a network statement.

• Use show ip protocols to review EIGRP network statements and filters.
• Use show ip eigrp topology to see whether the prefix was learned and whether it has a valid successor.
• Use show ip route on the advertising router to confirm that the destination exists there.
• Check distribute lists, prefix filters, route maps, or other route policies.
• Check whether a manual summary is intentionally hiding more-specific routes.
• Consider split horizon in hub-and-spoke or multipoint designs where routes may need to be advertised back through the same interface.

Correct the network statement or policy only after confirming the intended advertisement. Avoid disabling safeguards without understanding the topology.

Troubleshooting Unexpected Path Selection

If traffic uses a path that appears slower, inspect the metric components rather than judging the route only by hop count. Use:

show ip eigrp topology <destination-prefix>
show interfaces <interface>

Compare the minimum bandwidth and cumulative delay for each candidate path. An inaccurately configured bandwidth value can make a link appear better or worse than it really is. Correct interface bandwidth and delay only when the values accurately represent the link and the intended design; changing them merely to force a route can produce misleading routing behavior.

Configuration Hygiene and Design Considerations

• Use passive-interface default where appropriate, then explicitly enable EIGRP Hellos only on neighbor-facing interfaces.
• Avoid overly broad network statements when only selected interfaces should participate.
• Document the EIGRP AS number, interface addressing, summaries, authentication, filters, and intended neighbors.
• Use EIGRP authentication in controlled enterprise deployments to prevent unauthorized routers from forming adjacencies.
• Use route filtering deliberately. Filtering can prevent unwanted advertisements, but an incorrect filter can remove essential reachability.
• Save and review the configuration after testing.

Metric and Route-Selection Summary

Practical Validation Checklist

1. Confirm every routed interface has the expected IPv4 address and mask.
2. Confirm required interfaces are up/up.
3. Confirm the intended interfaces are matched by EIGRP network statements.
4. Confirm the EIGRP AS number matches between neighbors.
5. Confirm neighbor-facing interfaces are not passive.
6. Confirm the neighbor table contains the expected routers.
7. Confirm remote prefixes appear in the topology and routing tables.
8. Confirm the route's AD, metric, next hop, and outgoing interface.
9. Use ping between remote LAN addresses.
10. Use traceroute to verify the forwarding path.

Key Exam Notes

• EIGRP is an advanced distance-vector interior gateway protocol developed by Cisco.
• Hello packets discover and maintain directly connected neighbors.
• Classic EIGRP neighbors must use the same AS number and compatible K values.
• Default EIGRP metric inputs are bandwidth and delay; load and reliability are not used by default.
• The neighbor, topology, and routing tables serve different purposes.
• Internal EIGRP has a default AD of 90; external EIGRP has a default AD of 170.
• A passive interface advertises its connected network but does not send EIGRP Hellos.
• A network statement matches local interfaces and enables EIGRP on those interfaces.
• Manual summaries reduce routing-table entries but must cover the intended address space and be applied in the correct direction.
• Use show ip eigrp neighbors, show ip eigrp topology, and show ip route eigrp together when verifying operation.