EIGRP

Overview

EIGRP is Cisco's advanced distance-vector protocol using DUAL for loop-free backup paths. CCNA covers metrics, feasible successor, and basic configuration — awareness that it's less central than OSPF on the current exam.

EIGRP at a glance

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco advanced distance-vector protocol (often called hybrid). It combines distance-vector behaviors (initial route exchange, split horizon) with link-state-like features (topology table, incremental updates, DUAL loop prevention).

Proprietary (Cisco-centric at CCNA scope)
Protocol 88 over IP
Multicast to 224.0.0.10 (Hellos and updates)
Administrative distance: 90 (internal), 170 (external)
Supports VLSM and manual summarization
Default equal-cost load balancing: up to 4 paths (6 max configurable)

Info

EIGRP sends incremental updates when topology changes — not periodic full-table floods like classic RIP. The initial neighbor exchange still includes full routing information.

EIGRP tables

EIGRP maintains three structures per address family:

Table	Contents
Neighbor	Adjacent EIGRP routers — address, interface, uptime, SRTT
Topology	All learned destinations, feasible distance, reported distance, successors, feasible successors
Routing	Best routes (successors) installed for forwarding

EIGRP table verification

show ip eigrp neighbors show ip eigrp topology show ip eigrp topology all-links show ip route eigrp show ip protocols

DUAL — Diffusing Update Algorithm

DUAL is the loop-prevention engine. It selects a successor (best loop-free path) and may pre-compute a feasible successor (backup) before the primary fails — enabling fast convergence without counting to infinity.

Key terms:

Term	Definition
Reported Distance (RD)	Metric a neighbor advertises to reach a destination (also called advertised distance)
Feasible Distance (FD)	Your router's total metric to the destination (RD + cost to that neighbor)
Successor	Best route — lowest FD — installed in routing table
Feasible Successor	Backup route in topology table; must satisfy feasibility condition

Feasibility condition: A neighbor's RD to the destination must be less than your current FD. That proves the neighbor is not using you as its path to the destination — no loop if you switch to it.

EIGRP metric

Full formula (with K values):

Metric = [K1×BW + (K2×BW)/(256−Load) + K3×Delay] × [K5/(Reliability+K4)]

Defaults: K1=1, K2=0, K3=1, K4=0, K5=0 — only bandwidth and delay matter.

Simplified default:

Metric = (10⁷ ÷ minimum bandwidth in kbps) + (sum of delays in tens of µs) × 256

Bandwidth component: Based on slowest link in path (inverse bandwidth × 256)
Delay component: Sum of interface delays × 256
Load and reliability are dynamic — excluded by default to avoid constant recalculation

Tip

bandwidth and delay interface commands do not change physical link speed — they influence routing protocol metrics only.

Variance enables unequal-cost load balancing:

variance = highest alternate metric ÷ best metric (round up)

Example: if best FD = 3,014,400 and alternate = 3,847,680 → variance = 1.28 → configure variance 2.

Neighbor discovery and adjacency

Router sends Hello multicast to 224.0.0.10
Neighbor responds with Update (init bit set) — full routing info except routes learned on that interface (split horizon)
Both exchange Hellos → adjacency
ACKs confirm updates; routes enter topology table

Hello intervals:

Link type	Hello
High bandwidth (≥ T1)	5 sec
Low bandwidth multipoint	60 sec

Requirements for neighbors:

Matching AS number (autonomous system)
Matching K values (metric weights)
Same subnet on the link
Authentication password (if configured)

Basic EIGRP configuration

router eigrp 100 network 10.0.0.0 0.0.255.255 eigrp router-id 2.2.2.2 passive-interface Loopback0 maximum-paths 4

AS number: All EIGRP routers in the same domain must share the same AS (1–65535). Unlike OSPF process ID, this must match.

Topology table flags

When reading show ip eigrp topology:

Flag	Meaning
P	Passive — stable (good)
A	Active — DUAL computation in progress (investigate if stuck)
U	Update sent
Q	Query sent
R	Reply received

Load balancing

Equal-cost (default): Up to 4 paths with identical metric — maximum-paths 6 for six.

Unequal-cost: Requires variance multiplier — installs routes whose metric ≤ variance × best metric.

Path selection walkthrough

Three routers: R3 advertises destination with metric 5 to R2.

R2's RD from R3 = 5
R2's link to R3 cost = 10 → FD at R2 = 15
R2 advertises RD = 15 to R1
R1's link to R2 cost = 20 → FD at R1 = 35

R1 installs successor with FD 35. If a feasible successor exists (neighbor's RD < 35), it waits in the topology table for instant failover.

Passive interfaces

passive-interface stops Hellos in/out — no neighbors on that interface. Use on loopbacks and user-facing LANs where only a default gateway is needed.

passive-interface default
no passive-interface GigabitEthernet0/0

Troubleshooting

Issue	Check
Neighbors not forming	AS number, K values, subnet, authentication
Stuck in Active (SIA)	Query not answered — neighbor down, filter blocking
Unexpected path	Bandwidth/delay settings, offset lists, summarization
No EIGRP routes	Network statement wildcard, passive interface on transit link

Stuck in Active

Stuck-In-Active (SIA) queries indicate DUAL could not get replies — often a lost neighbor or filtered update. CCNA awareness: Active state should be transient.

EIGRP vs OSPF (exam comparison)

	EIGRP	OSPF
AD	90	110
Type	Advanced DV / hybrid	Link state
Metric	BW + delay (composite)	Cost (bandwidth)
Standards	Cisco proprietary	Open standard
Backup path	Feasible successor (precomputed)	Re-run SPF after LSU
Multicast	224.0.0.10	224.0.0.5 / .6

Exam checklist

EIGRP Router ID

EIGRP Router ID prevents routing loops for external routes — it is not used for neighbor identity the way OSPF RID is central to DR election.

Network command uses wildcard

network 10.0.0.0 0.0.255.255 under router eigrp uses a wildcard mask — same logic as OSPF and ACLs.

Quick review

DUAL picks successor (best FD) and feasible successor (backup meeting RD < FD)
RD = neighbor's advertised metric; FD = RD + cost to reach neighbor
Default metric uses slowest bandwidth + cumulative delay; K1 and K3 only by default
Neighbors require matching AS and K values; Hellos to 224.0.0.10
AD 90 beats OSPF's 110 when the same prefix competes
Verify: show ip eigrp neighbors, show ip eigrp topology, show ip route eigrp

How the sources were combined