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Adjacent Node
Networking, explained. No BS.

EIGRP

What It Is

Enhanced Interior Gateway Routing Protocol is a Cisco-originated interior routing protocol based on distance vector behavior with DUAL loop-free path selection. It is mostly a brownfield enterprise topic now. You still need to understand it when operating older Cisco networks, mergers, campus cores, WAN edge designs, and certification labs.

EIGRP can converge quickly and is straightforward in all-Cisco environments, but it is rarely the first choice for new multi-vendor designs. For new networks, compare it honestly against OSPF, IS-IS, and BGP.

Core Attributes

Attribute Value
Algorithm DUAL
Cisco administrative distance Internal 90, external 170, summary 5
Transport IP protocol 88
IPv4 multicast 224.0.0.10
IPv6 multicast ff02::a
Default K values K1 = 1, K3 = 1, others 0
Classic metric inputs Minimum bandwidth and cumulative delay by default
Modern metric note Named mode supports wide metrics

Modern note: RFC 7868 documents EIGRP as informational, not an Internet Standards Track protocol. Treat multi-vendor EIGRP as a risk unless it is explicitly tested.

Packet Types

Type Packet Purpose
1 Update Advertises route changes
3 Query Asks neighbors for alternate paths
4 Reply Answers a query
5 Hello Discovers and maintains neighbors
8 Acknowledgment Confirms reliable packet delivery

DUAL Terms

Term Meaning
Successor Best next hop installed in the routing table
Feasible successor Backup path that satisfies loop-free feasibility
Feasible distance Local best metric to reach a prefix
Reported distance Neighbor's advertised distance to the prefix
Feasibility condition Neighbor reported distance is lower than local feasible distance
Passive route Stable route, not actively recomputing
Active route Route is querying for a new loop-free path
Stuck in active Query process did not complete before timeout

Watch out: "Passive" is good in the topology table. It means stable. A passive interface is a separate configuration concept.

Design Choices

Choice Recommendation
Classic vs named mode Prefer named mode for modern IOS/IOS-XE work
Summarization Summarize at boundaries to limit query scope
Stub routing Use on spokes and access routers that should not be transit
Passive interfaces Use passive by default, enable only routing adjacencies
K values Do not change unless every router matches and the reason is documented
Unequal-cost load sharing Use variance carefully and verify traffic split
Authentication Use supported authentication for trusted adjacencies
Redistribution Tag and filter redistributed routes

Design note: EIGRP scales better when queries are contained. Stub routing, summarization, and clean topology matter more than clever metrics.

Cisco IOS/IOS-XE Examples

Named mode IPv4:

router eigrp CAMPUS
 address-family ipv4 unicast autonomous-system 100
  eigrp router-id 10.255.0.10
  af-interface default
   passive-interface
  exit-af-interface
  af-interface GigabitEthernet1/0/1
   no passive-interface
   authentication mode md5
   authentication key-chain EIGRP-KEYS
  exit-af-interface
  network 10.0.0.0 0.255.255.255
  topology base
   variance 1

Stub spoke:

router eigrp BRANCH
 address-family ipv4 unicast autonomous-system 100
  eigrp stub connected summary
  network 10.50.0.0 0.0.255.255

Manual summary:

interface GigabitEthernet1/0/1
 ip summary-address eigrp 100 10.50.0.0 255.255.0.0

Notes:

  • Syntax differs between classic and named mode. Do not mix examples without checking the platform.
  • Named mode is the better default for new IOS-XE EIGRP configurations.
  • Keep autonomous system numbers, K values, authentication, and address families consistent across neighbors.

Troubleshooting

Symptom Check Likely Cause
No neighbor AS number, subnet, passive interface, ACL, multicast Hellos not exchanged
Neighbor flaps Hold timer, loss, interface errors, CPU Missed hellos
Route missing Topology table, stub, summary, filter Query boundary or policy
Route stuck active Query scope, unreachable neighbor, summarization Unbounded query domain
Bad path chosen Bandwidth, delay, wide metric, offset list Metric mismatch
External route surprise Redistribution tags and route maps Route leaked or reintroduced
Unequal load not working Variance and feasibility condition Candidate is not feasible

Commands

show ip eigrp neighbors
show ip eigrp interfaces
show ip eigrp topology
show ip eigrp topology all-links
show ip eigrp traffic
show ip protocols
show ip route eigrp
clear ip eigrp neighbors

Expected clues:

  • Neighbor uptime is stable.
  • Topology table has a successor for expected prefixes.
  • Feasible successors exist where backup paths are expected.
  • Stubs and summaries limit query propagation.
  • EIGRP traffic counters move during adjacency and route changes.

Watch Out

  • Do not change K values casually.
  • Do not run EIGRP on user-facing interfaces.
  • Do not redistribute between EIGRP and another protocol without tags and filters.
  • Do not ignore SIA events. They are topology and query-scope problems, not just log noise.
  • Do not assume RFC publication means practical multi-vendor interoperability.

References