EIGRP Complete Guide

EIGRP foundations

EIGRP is often described as an advanced distance-vector protocol because routers advertise distance and next-hop information while also maintaining a topology table. Unlike a simple distance-vector protocol, it does not periodically resend the entire routing table. It sends bounded updates when relevant information changes.

Component	Purpose
Neighbour table	Records directly connected EIGRP peers that have passed compatibility checks.
Topology table	Stores routes learned from neighbours, including feasible distances and reported distances.
Routing table	Contains the best successor routes installed for forwarding.
DUAL	Calculates successors and feasible successors while maintaining loop-free operation.

Classic EIGRP uses IP protocol number 88. IPv4 multicast hellos are normally sent to 224.0.0.10; IPv6 EIGRP uses FF02::A. Exact timers and behaviour depend on interface type and implementation.

Neighbour formation and packet types

Two routers become neighbours only when key parameters are compatible. They must be directly reachable, use the same autonomous-system process, agree on the primary metric components, and pass authentication when it is configured. The IP subnets must also permit direct communication.

Packet	Role	Reliability
Hello	Discovers and maintains neighbours.	Unreliable
Update	Carries routing information.	Reliable when acknowledgement is required
Query	Asks neighbours for an alternate route during active computation.	Reliable
Reply	Answers a query.	Reliable
Acknowledgement	Confirms receipt of a reliable packet.	Unreliable packet carrying an acknowledgement number

Interview point: EIGRP uses RTP, the Reliable Transport Protocol defined for EIGRP. This is unrelated to the Real-time Transport Protocol used for voice and video.

DUAL terminology and feasible successors

The successor is the next hop for the lowest-metric path and is normally installed in the routing table. A feasible successor is a verified loop-free backup path kept in the topology table. It can be used immediately if the successor fails.

Reported distance (RD): the neighbour's own metric to the destination.
Feasible distance (FD): the local router's best known metric to the destination.
Feasibility condition: a neighbour can be a feasible successor when its reported distance is strictly lower than the local feasible distance.
Passive state: the route is stable; passive is healthy in EIGRP.
Active state: DUAL is querying neighbours because no immediate feasible successor is available.

If replies do not return before the active timer expires, the route can become stuck-in-active. Modern troubleshooting should identify the missing reply, slow link, excessive query scope, unstable neighbour or overloaded device rather than merely increasing timers.

Composite metric and route selection

The default classic metric is based mainly on the minimum bandwidth and cumulative delay along the path. Reliability and load exist as metric components but normally have zero K-values and therefore do not influence the calculation. MTU is carried in routing information but is not part of the path metric.

Common mistake: The bandwidth command does not physically change interface speed. It supplies a reference value to routing and operational features. Incorrect bandwidth or delay values can produce an unexpected path.

Equal-cost paths can be installed together. EIGRP can also perform unequal-cost load sharing with the variance feature, but only paths that satisfy the feasibility condition are eligible. The maximum-paths setting limits how many paths enter the routing table.

Configuration and verification

Cisco IOS-style named EIGRP example

router eigrp CAMPUS
 address-family ipv4 unicast autonomous-system 100
  network 10.10.0.0 0.0.255.255
  passive-interface default
  no passive-interface GigabitEthernet0/0
 exit-address-family

Useful verification

show ip eigrp neighbors
show ip eigrp topology
show ip route eigrp
show ip protocols
show interfaces
show logging

A route code of D normally represents an internal EIGRP route and D EX an external route redistributed into EIGRP. Administrative-distance defaults are vendor-specific; on Cisco IOS, internal and external EIGRP routes traditionally use different values.

Troubleshooting workflow

Verify interface status, addressing and direct IP reachability.
Confirm the same EIGRP autonomous-system number and address family.
Check whether the interface is passive or excluded by the network statement.
Compare K-values and authentication parameters.
Inspect neighbour uptime, sequence numbers and retransmissions.
Check the topology table for active routes and outstanding replies.
Validate bandwidth, delay, summarisation, filtering and redistribution policy.
Check CPU, packet loss, MTU and access-control rules for protocol 88.

Interview focus

Use these points to practise concise, operations-focused answers. Explain the concept first, then give a verification or troubleshooting example.

What does passive mean in the EIGRP topology table? The route is stable and DUAL is not actively querying for a replacement path.
What is a feasible successor? A loop-free backup next hop whose reported distance satisfies the feasibility condition.
Which two values normally influence the default EIGRP metric? Minimum bandwidth and cumulative delay.
Why might two EIGRP routers fail to become neighbours? Examples include different AS numbers, K-values, authentication, passive interfaces, filtering, addressing or failed Layer 2 connectivity.
What causes a route to enter the active state? The successor is lost and no immediately usable feasible successor exists, so DUAL sends queries.

Quick self-check

1. What does passive mean in the EIGRP topology table?

Answer: The route is stable and DUAL is not actively querying for a replacement path.

2. What is a feasible successor?

Answer: A loop-free backup next hop whose reported distance satisfies the feasibility condition.

3. Which two values normally influence the default EIGRP metric?

Answer: Minimum bandwidth and cumulative delay.

4. Why might two EIGRP routers fail to become neighbours?

Answer: Examples include different AS numbers, K-values, authentication, passive interfaces, filtering, addressing or failed Layer 2 connectivity.

5. What causes a route to enter the active state?

Answer: The successor is lost and no immediately usable feasible successor exists, so DUAL sends queries.

Frequently asked questions

1. Is EIGRP an open standard?

Answer: Its core protocol behaviour was documented in RFC 7868 as an informational specification. Feature support and configuration still vary by vendor.

2. Does EIGRP send full updates periodically?

Answer: No. It normally sends partial, bounded updates when topology information changes.

3. Can EIGRP load-balance across unequal paths?

Answer: Yes, implementations such as Cisco IOS can use variance, subject to the feasibility condition and path limits.

Standards and further reading

Editorial note: Standards explain protocol behaviour, while commands and defaults can differ by vendor, platform and software release. Verify production changes against current vendor documentation.