EIGRP Overview: Concepts, Operation, and Basic Configuration

Learn EIGRP concepts, DUAL route selection, neighbors, metrics, tables, classic IPv4 configuration, verification, summarization, and troubleshooting on Cisco IOS.

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-developed dynamic interior gateway routing protocol (IGP). An IGP exchanges routing information within one autonomous system, such as an organization's internal network. EIGRP learns reachable networks, selects paths, reacts to failures, and maintains neighbor relationships with other EIGRP routers.

This lesson assumes familiarity with the OSI reference model, IPv4 addressing, subnet masks, Cisco IOS modes, connected routes, and basic ping and traceroute testing.

EIGRP Purpose and Classification

EIGRP is commonly described as an advanced distance-vector protocol. It uses information from neighboring routers, but its route-selection and convergence behavior is more sophisticated than traditional distance-vector protocols.

Interior protocol: It is designed to exchange routes inside one autonomous system.
Protocol number 88: EIGRP is carried directly in IP; it does not use TCP or UDP port numbers.
Partial and bounded updates: Routers normally send only changed information to affected neighbors rather than periodically sending their entire routing table.
Triggered updates: A topology change causes relevant information to be sent promptly.
Rapid convergence: EIGRP can use a prequalified backup route when one exists.
Unequal-cost load balancing: With the variance setting, eligible paths with different metrics can share traffic.

Classic EIGRP uses a router configuration process such as router eigrp 100, where the number is the autonomous-system (AS) number. Named-mode EIGRP uses a named configuration hierarchy and supports multiple address families and additional structured settings. The fundamental ideas—neighbors, metrics, DUAL, and topology information—remain related, but the commands differ.

Characteristic	Value or behavior	Why it matters
IP protocol number	88	EIGRP is not a TCP or UDP application.
IPv4 multicast address	224.0.0.10	IPv4 EIGRP control traffic can reach EIGRP routers on the local segment.
Default metric inputs	Minimum bandwidth and cumulative delay	Lower composite metrics are preferred.
Internal administrative distance	90	Determines trust relative to routes from other sources.
External administrative distance	170	Redistributed EIGRP routes are less preferred by default than internal EIGRP routes.
Update behavior	Partial, triggered, and bounded	Reduces unnecessary update traffic.
Loop-prevention mechanism	DUAL and the feasibility condition	Supports loop-free route selection and convergence.

Protocol Operation

EIGRP sends IPv4 control traffic to multicast address 224.0.0.10. It uses the Reliable Transport Protocol (RTP), an EIGRP transport mechanism that provides sequencing and dependable delivery for selected messages. RTP is not the same as the multimedia protocol that commonly uses the same acronym.

Hello packets discover neighbors and maintain existing adjacencies. They do not normally require reliable delivery acknowledgments.
Updates advertise route information. Updates are sent reliably when delivery must be confirmed.
Queries ask neighbors for an alternative path when a route has failed and no usable backup is available.
Replies answer queries.
Acknowledgments confirm receipt of reliable EIGRP messages.

Hello packets are sent periodically. Each neighbor also advertises a hold time, the maximum time the receiving router should wait without hearing another hello. If the hold timer expires, the router removes the neighbor adjacency and recalculates routes that depended on it.

EIGRP updates are partial because they contain changed route information, triggered because changes cause them to be sent, and bounded because they are normally sent only to neighbors affected by the change. This is more efficient than repeatedly flooding complete routing tables.

Neighbor Discovery and Adjacency Requirements

Two routers form an EIGRP adjacency when they are directly connected at Layer 3 and successfully exchange compatible hello information. In classic EIGRP, the following conditions must be satisfied:

The interfaces must be operational and share a common Layer 3 network.
The EIGRP autonomous-system numbers must match.
The metric K values must match.
Authentication settings must be compatible when authentication is enabled.
The address-family configuration must be compatible.
Neither required interface should be blocked by an ACL, incorrectly subnetted, or configured as passive.

Timers do not always have to be identical for an adjacency to form, but incompatible or improperly changed hello and hold-time behavior can cause resets or premature neighbor loss. MTU problems can also prevent reliable adjacency formation or cause instability even when basic IP reachability works.

DUAL, Metrics, and Route States

The Diffusing Update Algorithm (DUAL) selects loop-free paths and responds to topology changes. EIGRP stores candidate paths in its topology table before deciding which paths belong in the routing table.

A successor is the preferred next-hop path to a destination. Its route is normally installed in the routing table.
A feasible successor is a prequalified, loop-free backup path stored in the topology table. It can replace the successor without a full query process when the primary path fails.
Feasible distance (FD) is the local router's best calculated metric to a destination, including the path to the selected neighbor.
Reported distance (RD), also called advertised distance, is the metric a neighbor reports for reaching that destination.
The feasibility condition is satisfied when the neighbor's RD is lower than the local router's FD. That neighbor can then qualify as a feasible successor.

A route in passive state is stable; passive does not mean that an interface is configured with the passive-interface command. An active route is being recalculated. EIGRP enters active state when it needs to query neighbors for a replacement path. A route that remains active for too long indicates a convergence or query-scope problem.

Term	Definition	Relationship to route selection
Successor	Preferred loop-free next hop	Normally installed in the routing table.
Feasible successor	Backup path meeting the feasibility condition	Can provide rapid failover.
Feasible distance	Local best metric to a destination	Reference value for feasibility.
Reported distance	Neighbor's advertised metric	Must be lower than local FD for backup eligibility.
Feasibility condition	RD < local FD	Prevents a qualifying backup from forming a routing loop.
Passive state	Stable route state	No replacement-path query is in progress.
Active state	Route is being queried	Indicates DUAL is searching for a new path.

EIGRP Metric Calculation

EIGRP calculates a composite metric. With the default settings, the important inputs are the minimum bandwidth along the path and the cumulative delay. Higher bandwidth generally lowers the metric, while greater delay raises it. EIGRP prefers the path with the lower metric.

The metric uses weighting constants called K values. The full formula can include bandwidth, load, delay, and reliability, but the default configuration enables only K1 and K3:

K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0

Thus, load and reliability do not affect the default metric. They participate only if their associated K values are changed. Changing K values is generally avoided because every EIGRP neighbor in the domain must use matching values; a mismatch prevents adjacency formation.

EIGRP Tables and Route Selection

Table	What it stores	Typical verification command	Forwarding role
Neighbor table	Directly connected routers with formed adjacencies	`show ip eigrp neighbors`	Provides next-hop and adjacency information.
Topology table	Learned EIGRP destinations, metrics, successors, and candidate paths	`show ip eigrp topology`	Supplies candidates to DUAL.
Routing table	Best routes selected from all routing sources	`show ip route`	Used to forward packets.

Information typically progresses from a neighbor's advertisement, to a topology-table entry, to DUAL's successor decision, and finally to routing-table installation. EIGRP internal routes have a default administrative distance of 90. EIGRP external routes have a default administrative distance of 170.

Internal and External EIGRP Routes

An internal EIGRP route originates within the EIGRP routing domain. Cisco IOS commonly displays its route code as D. An external EIGRP route enters EIGRP through redistribution or another external injection mechanism and is commonly displayed as D EX.

External information includes details about the originating routing source and metric information. Redistribution must be planned carefully: route filtering, redistribution direction, tagging, and policy controls help prevent loops and unintended route feedback.

Basic Classic EIGRP for IPv4 Configuration

The following example enables classic EIGRP process 100, sets a router ID, activates EIGRP on one exact point-to-point address and one LAN subnet, makes the user-facing LAN passive, and disables automatic classful summarization.

R1(config)# router eigrp 100
R1(config-router)# eigrp router-id 1.1.1.1
R1(config-router)# network 10.0.12.1 0.0.0.0
R1(config-router)# network 192.168.10.0 0.0.0.255
R1(config-router)# passive-interface GigabitEthernet0/0
R1(config-router)# no auto-summary

A network statement matches local interfaces. Matching an interface activates EIGRP on that interface and causes its connected network to be advertised. The wildcard mask identifies which address bits must match: a 0 means “must match,” while a 1 means “ignore.” Therefore, 10.0.12.1 0.0.0.0 matches exactly one interface address, while 192.168.10.0 0.0.0.255 matches addresses in that /24 range.

The same AS number must be configured on routers that should become classic EIGRP neighbors. A router ID is a 32-bit identifier used to identify the EIGRP router; configuring it explicitly makes the identity predictable. The passive-interface command suppresses EIGRP hello packets on an interface but normally retains advertisement of the connected subnet. Use it on user-facing LANs, not on an inter-router link that requires an adjacency. no auto-summary prevents older automatic classful summarization behavior from creating inappropriate summaries in discontiguous or classless networks.

Two-Router Adjacency Example

Suppose R1 and R2 use 10.0.12.0/30 on their point-to-point link. Configure EIGRP on both link interfaces with AS 100:

R1(config)# router eigrp 100
R1(config-router)# eigrp router-id 1.1.1.1
R1(config-router)# network 10.0.12.1 0.0.0.0
R1(config-router)# no auto-summary

R2(config)# router eigrp 100
R2(config-router)# eigrp router-id 2.2.2.2
R2(config-router)# network 10.0.12.2 0.0.0.0
R2(config-router)# no auto-summary

After hello exchange, each router should list the other in show ip eigrp neighbors. In a three-router line such as R1—R2—R3, R2 can learn the LAN behind R1 and the LAN behind R3, select successors, and install those remote prefixes in its routing table.

Verification and Monitoring

show ip protocols
show ip eigrp neighbors
show ip eigrp topology
show ip eigrp topology all-links
show ip route eigrp
show ip route
show ip eigrp interfaces detail
ping <destination>
traceroute <destination>

show ip protocols verifies the EIGRP process, AS number, router ID, network statements, passive interfaces, and automatic summarization setting.
show ip eigrp neighbors displays neighbor address, local interface, hold time, uptime, and the neighbor's queue and sequence information.
show ip eigrp topology shows learned prefixes, metrics, route state, successors, and eligible backup information. all-links helps expose paths that are not currently successors.
show ip route eigrp displays EIGRP-installed routes. Look for D and D EX.
Ping tests reachability; traceroute tests the forwarding path and can reveal an unexpected next hop.

Use debugging sparingly:

debug eigrp packets hello
undebug all

Production caution: Debug output can consume CPU, memory, console bandwidth, and operator attention. Use it only in a controlled environment or maintenance window, and stop it promptly with undebug all.

Load Balancing and Summarization

EIGRP can install multiple equal-cost successor paths and distribute traffic across them. Unequal-cost load balancing uses variance to permit eligible paths whose metrics differ from the best path:

R1(config)# router eigrp 100
R1(config-router)# variance 2

The alternate path must satisfy the feasibility condition and must fall within the variance threshold. Variance does not make an unsafe path eligible. Verify the resulting next hops with show ip route. The number of installed paths can also be affected by the maximum-paths setting.

Manual summarization combines multiple contiguous prefixes into one advertised summary on a specific interface:

R1(config)# interface GigabitEthernet0/1
R1(config-if)# ip summary-address eigrp 100 192.168.0.0 255.255.252.0

Apply the summary outbound toward the receiving neighbor. Summarization can reduce routing-table size, reduce update traffic, and contain topology changes. The router may create a discard route for the summary so that traffic matching the summary but not a known more-specific route is dropped rather than looped. Verify that the summary covers only intended networks. Manual summarization is different from automatic summarization, which derives classful boundaries and is normally disabled in modern classless designs.

Common Adjacency Failures

Symptom	Likely cause	Verification approach	Corrective action
No neighbor entry	Interface down, wrong network statement, ACL, subnetting, or passive interface	`show ip interface brief`, `show ip protocols`, interface and ping checks	Restore Layer 3 reachability, correct the match, permit control traffic, or remove passive mode where needed.
Neighbors reset repeatedly	Unstable interface, timers, MTU, authentication, or packet loss	Inspect interface counters, logs, timers, MTU, and authentication	Correct the link or make settings compatible on both routers.
K-value mismatch	Different metric weighting constants	Compare EIGRP metric settings	Use matching K values; normally restore defaults.
AS-number mismatch	Different classic EIGRP process numbers	Run `show ip protocols` on both routers	Use the same AS number on the intended adjacency.
Interface configured as passive	Hello packets suppressed	Check passive-interface output	Remove passive mode from a required neighbor link.
Authentication mismatch	Different key, mode, or key chain	Compare interface authentication configuration	Make authentication settings and keys compatible.
Layer 3 addressing failure	Different subnet, incorrect mask, or failed interface	Check addresses, masks, status, and targeted ping	Correct addressing or restore the interface.

Operational Troubleshooting

No Neighbor Forms

Confirm both interfaces are up/up and have addresses in the same subnet.
Verify EIGRP is enabled on both link interfaces.
Compare classic AS numbers, K values, authentication, and relevant timers.
Ensure neither link interface is passive.
Check ACLs, MTU, packet loss, and hello traffic only after basic checks.

A Remote Network Is Missing

Confirm the advertising router knows the network through a connected or other valid route.
Verify the correct interface is matched by an EIGRP network statement.
Confirm a working neighbor relationship along the path.
Inspect the topology table to determine whether the prefix is learned but not selected.
Check filtering, summarization, redistribution, and competing routes.

A Route Is Active or Convergence Is Slow

Inspect the topology entry for outstanding queries, failed neighbors, and the presence or absence of a feasible successor. Excessive query scope often indicates a flat design or insufficient summarization. Resolve failed adjacencies and use hierarchical addressing and appropriate summaries to limit query propagation.

Variance Does Not Install the Alternate Path

Verify the variance value, compare the alternate metric with the allowed threshold, and confirm the alternate satisfies the feasibility condition. Check maximum-paths if more paths are expected. Do not force traffic onto a path that is not loop-free or operationally appropriate.

Summarization Produces Incorrect Results

Validate the summary prefix and mask, confirm the direction of the interface configuration, inspect any summary discard route, and ensure that required more-specific routes still exist. Correct the summary boundaries or remove an inappropriate summary, then retest with ping and traceroute.

Design Considerations and Exam Notes

Remember: EIGRP uses IP protocol 88 and multicast 224.0.0.10 for IPv4 control traffic.
Default K values are K1=1, K2=0, K3=1, K4=0, and K5=0.
Default administrative distances are 90 for internal EIGRP and 170 for external EIGRP.
D identifies an internal EIGRP route; D EX identifies an external EIGRP route.
A passive route is stable; a passive interface suppresses hello packets. These are different concepts.
Passive interfaces are useful on user LANs, but not on links where EIGRP neighbors are required.
Route filtering and redistribution are advanced features. Use explicit policy and careful planning to avoid loops, route feedback, and unexpected path selection.

For related routing practice, compare this workflow with basic OSPF configuration and review computer networking fundamentals.