EIGRP Configuration

EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco advanced distance-vector routing protocol that combines the fast convergence of link-state protocols with the simplicity of distance-vector operation. Unlike OSPF, which floods the entire topology to all routers, EIGRP shares routing information only with directly connected neighbors — reducing bandwidth and processing overhead while still converging much faster than older protocols like RIP. For an overview of EIGRP concepts see EIGRP Overview.

EIGRP's key advantage is its DUAL (Diffusing Update Algorithm) — a loop-free path selection algorithm that pre-calculates backup paths before they are needed. When a primary path fails, EIGRP can switch to a pre-validated backup path (feasible successor) in milliseconds — without triggering a network-wide recalculation. Originally Cisco-proprietary, EIGRP was opened as an informational RFC in 2013 (RFC 7868).

Before starting, complete OSPF Single-Area Configuration and Static Route Configuration to understand routing fundamentals and Administrative Distance comparisons. For a routing protocol comparison including BGP see BGP Overview and Administrative Distance.

1. EIGRP — Core Concepts

EIGRP vs OSPF — Key Differences

Feature	EIGRP	OSPF
Type	Advanced distance-vector (hybrid)	Link-state
Algorithm	DUAL (Diffusing Update Algorithm)	Dijkstra SPF
Metric	Composite: bandwidth + delay (+ load, reliability optionally)	Cost (reference bandwidth / link bandwidth)
Administrative Distance	Internal: 90 / External: 170	110
Hello multicast	224.0.0.10	224.0.0.5 / 224.0.0.6
Hello / Hold timers (Ethernet)	5 sec / 15 sec	10 sec / 40 sec
Areas / Boundaries	No areas — flat topology (single AS)	Hierarchical areas — Area 0 backbone required
Partial updates	Only sends updates when topology changes	Full LSDB exchange on adjacency, then partial LSAs
Unequal-cost load balancing	✅ Yes — via `variance` command	❌ No — equal cost only (ECMP)

EIGRP Terminology — The Three Tables

Table	Command	Contains
Neighbor Table	`show ip eigrp neighbors`	All directly connected EIGRP routers that have formed adjacency. Populated by Hello packets.
Topology Table	`show ip eigrp topology`	All routes learned from all neighbors — including successors and feasible successors. The DUAL algorithm runs on this table.
Routing Table	`show ip route eigrp`	Only the best routes (successors) installed for forwarding. Marked D for EIGRP.

EIGRP Metric — FD, AD, Successor, Feasible Successor

EIGRP uses two metric values per route, calculated from each neighbor's perspective:

Term	Abbreviation	Definition
Feasible Distance	FD	The total metric cost from this router to the destination — the end-to-end cost including the link to the neighbor
Reported Distance	RD (also called AD — Advertised Distance)	The metric cost as reported by a neighbor — the cost from the neighbor to the destination (not including the link to reach the neighbor)
Successor	S	The neighbor providing the lowest FD path to a destination — installed in the routing table as the primary route
Feasible Successor	FS	A backup neighbor whose RD is less than the Successor's FD — the Feasibility Condition. Stored in topology table, instantly promoted if Successor fails

The Feasibility Condition: A neighbor qualifies as a Feasible Successor only if its Reported Distance (RD) is strictly less than the current Successor's Feasible Distance (FD). This guarantees the backup path is loop-free — if the backup path's reported cost is already lower than the primary's total cost, the backup cannot loop back through this router.

EIGRP Metric Calculation

EIGRP's composite metric uses bandwidth and delay by default:

Metric = 256 × ( (10^7 / minimum-bandwidth-kbps) + (sum-of-delays / 10) )

K Value	Component	Default	Notes
K1	Bandwidth	1 (enabled)	Uses the minimum bandwidth along the entire path
K2	Load	0 (disabled)	Interface utilization — not used by default (unstable metric)
K3	Delay	1 (enabled)	Cumulative delay across all links in the path (in tens of microseconds)
K4 / K5	Reliability	0 (disabled)	Not used by default

K values must match between neighbors. If R1 has K1=1, K3=1 and R2 has K1=1, K2=1, K3=1, they will not form an EIGRP adjacency. K values are included in Hello packets and must be identical on both sides.

2. Lab Topology & IP Addressing

Three routers in a hub-and-spoke arrangement. R1 is the hub connected to both R2 and R3. R2 and R3 also connect to each other — giving EIGRP multiple paths to evaluate for successor and feasible successor selection.

  192.168.10.0/24                                    192.168.30.0/24
  (LAN — PC1)                                        (LAN — PC3)
       |                                                    |
    Gi0/0                                               Gi0/0
  ┌─────────┐  10.0.12.0/30   ┌─────────┐  10.0.23.0/30  ┌─────────┐
  │   R1    │──Gi0/1──────────│   R2    │──Gi0/1──────────│   R3    │
  │ RID:    │                 │ RID:    │                 │ RID:    │
  │ 1.1.1.1 │──Gi0/2──────────│ 2.2.2.2 │                 │ 3.3.3.3 │
  └─────────┘  10.0.13.0/30   └─────────┘                 └─────────┘
                               Gi0/2                       Gi0/1
                              (to R3 via 10.0.23.0/30)

  EIGRP AS Number: 100

Device	Interface	IP Address	Connected To
NetsTuts_R1	Gi0/0	192.168.10.1 /24	LAN (PC1)
NetsTuts_R1	Gi0/1	10.0.12.1 /30	NetsTuts_R2 Gi0/0
NetsTuts_R1	Gi0/2	10.0.13.1 /30	NetsTuts_R3 Gi0/0
NetsTuts_R2	Gi0/0	10.0.12.2 /30	NetsTuts_R1 Gi0/1
NetsTuts_R2	Gi0/1	192.168.20.1 /24	LAN (PC2)
NetsTuts_R2	Gi0/2	10.0.23.1 /30	NetsTuts_R3 Gi0/1
NetsTuts_R3	Gi0/0	10.0.13.2 /30	NetsTuts_R1 Gi0/2
NetsTuts_R3	Gi0/1	10.0.23.2 /30	NetsTuts_R2 Gi0/2
NetsTuts_R3	Gi0/2	192.168.30.1 /24	LAN (PC3)

3. Step 1 — Configure EIGRP on NetsTuts_R1

EIGRP is configured under a single process using an Autonomous System (AS) number — all routers in the same EIGRP domain must use the same AS number. Unlike OSPF's process ID, the EIGRP AS number must match between neighbors for adjacency to form. For basic interface configuration prerequisites see Basic Interface Configuration. After completing EIGRP configuration, save with write memory.

NetsTuts_R1>en
NetsTuts_R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.

! ── Start EIGRP AS 100 ────────────────────────────────────
NetsTuts_R1(config)#router eigrp 100

! ── Set Router ID manually ────────────────────────────────
NetsTuts_R1(config-router)#eigrp router-id 1.1.1.1

! ── Disable auto-summary (best practice) ─────────────────
NetsTuts_R1(config-router)#no auto-summary

! ── Advertise all networks ────────────────────────────────
NetsTuts_R1(config-router)#network 192.168.10.0 0.0.0.255
NetsTuts_R1(config-router)#network 10.0.12.0 0.0.0.3
NetsTuts_R1(config-router)#network 10.0.13.0 0.0.0.3

! ── Suppress Hellos on LAN interface ─────────────────────
NetsTuts_R1(config-router)#passive-interface GigabitEthernet0/0
NetsTuts_R1(config-router)#exit
NetsTuts_R1(config)#end
NetsTuts_R1#wr
Building configuration...
[OK]
NetsTuts_R1#

Key EIGRP Configuration Commands

Command	What It Does	Notes
`router eigrp [AS]`	Starts the EIGRP process for the specified AS number	AS number must match on all routers in the same EIGRP domain
`eigrp router-id [IP]`	Manually sets the EIGRP Router ID	Best practice — prevents RID changes if interfaces go up/down. Same selection logic as OSPF (manual > loopback > physical)
`no auto-summary`	Disables automatic classful summarization	Critical — auto-summary is enabled by default in older IOS and causes routing black holes in discontiguous networks. Always disable it.
`network [IP] [wildcard]`	Enables EIGRP on interfaces matching the network/wildcard and advertises the network	Same wildcard mask logic as OSPF. No area number required.
`passive-interface [int]`	Suppresses Hello packets on the interface but still advertises the network	Same behavior as OSPF passive-interface — use on all LAN-facing ports

4. Step 2 — Configure EIGRP on NetsTuts_R2 and NetsTuts_R3

NetsTuts_R2

NetsTuts_R2>en
NetsTuts_R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
NetsTuts_R2(config)#router eigrp 100
NetsTuts_R2(config-router)#eigrp router-id 2.2.2.2
NetsTuts_R2(config-router)#no auto-summary
NetsTuts_R2(config-router)#network 10.0.12.0 0.0.0.3
NetsTuts_R2(config-router)#network 10.0.23.0 0.0.0.3
NetsTuts_R2(config-router)#network 192.168.20.0 0.0.0.255
NetsTuts_R2(config-router)#passive-interface GigabitEthernet0/1
NetsTuts_R2(config-router)#exit
NetsTuts_R2(config)#end
NetsTuts_R2#wr
Building configuration...
[OK]
NetsTuts_R2#
%DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 10.0.12.1 (GigabitEthernet0/0) is up: new adjacency

IOS displays the adjacency message when R2 forms a neighbor relationship with R1 (10.0.12.1). DUAL-5-NBRCHANGE is the EIGRP equivalent of OSPF's ADJCHG message — it confirms the new adjacency is operational.

NetsTuts_R3

NetsTuts_R3>en
NetsTuts_R3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
NetsTuts_R3(config)#router eigrp 100
NetsTuts_R3(config-router)#eigrp router-id 3.3.3.3
NetsTuts_R3(config-router)#no auto-summary
NetsTuts_R3(config-router)#network 10.0.13.0 0.0.0.3
NetsTuts_R3(config-router)#network 10.0.23.0 0.0.0.3
NetsTuts_R3(config-router)#network 192.168.30.0 0.0.0.255
NetsTuts_R3(config-router)#passive-interface GigabitEthernet0/2
NetsTuts_R3(config-router)#exit
NetsTuts_R3(config)#end
NetsTuts_R3#wr
Building configuration...
[OK]
NetsTuts_R3#
%DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 10.0.13.1 (GigabitEthernet0/0) is up: new adjacency
%DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 10.0.23.1 (GigabitEthernet0/1) is up: new adjacency

R3 forms adjacency with both R1 (via Gi0/0) and R2 (via Gi0/1) simultaneously. Two NBRCHANGE messages confirm both relationships are established. All three routers now have full EIGRP adjacency.

5. DUAL Algorithm — Successor and Feasible Successor

With all three routers running EIGRP, consider how R1 selects its path to 192.168.30.0/24 (PC3's network on R3). R1 has two possible paths: directly to R3 via Gi0/2, or via R2 through the 10.0.23.0/30 link. DUAL evaluates both using FD and RD:

  R1 calculating best path to 192.168.30.0/24:

  Path 1: R1 ──Gi0/2──► R3 (direct)
    R3's Reported Distance (RD) to 192.168.30.0/24 = 2816
    R1's cost to reach R3 via Gi0/2             = 2816
    R1's Feasible Distance (FD)                 = 5632   ← LOWER → SUCCESSOR

  Path 2: R1 ──Gi0/1──► R2 ──► R3 (via R2)
    R2's Reported Distance (RD) to 192.168.30.0/24 = 5632
    R1's cost to reach R2 via Gi0/1              = 2816
    R1's Feasible Distance (FD)                  = 8448

  Feasibility Condition check for Path 2 as Feasible Successor:
    R2's RD (5632) < Successor's FD (5632)?  NO — 5632 is NOT less than 5632
    → Path 2 does NOT qualify as Feasible Successor
    → DUAL must run a full diffusing computation if Path 1 fails

Why the Feasibility Condition is strict (< not ≤): If a neighbor's RD equals the Successor's FD, it is possible the neighbor routes back through this router — creating a potential loop. Only an RD strictly less than the current FD guarantees the backup path is loop-free. This is the mathematical foundation of DUAL.

What Happens When the Successor Fails

Scenario	Feasible Successor Exists?	EIGRP Response	Convergence Time
Primary link (Successor) fails	✅ Yes	Feasible Successor is instantly promoted to Successor — no queries sent	Milliseconds — no recalculation needed
Primary link fails, no FS	❌ No	Route enters Active state — DUAL sends Query packets to all neighbors asking for an alternative path	Seconds — depends on network size and query scope
All paths fail	❌ No paths	Route is removed from topology and routing tables	Route removed after Hold timer expires

Active vs Passive Route States

State	Meaning	Good?
Passive (P)	Route is stable — Successor is known and installed in routing table	✅ Normal operating state
Active (A)	Route is undergoing DUAL recalculation — Queries have been sent to neighbors, awaiting Reply packets	⚠️ Transitional — if stuck, indicates SIA (Stuck In Active)

6. EIGRP Summarization and Variance

Manual Summarization

Unlike OSPF (which summarises at the ABR), EIGRP summarization is configured directly on the outgoing interface using ip summary-address eigrp [AS] [network] [mask]. EIGRP also automatically installs a Null0 discard route for the summary to prevent loops. For route summarisation techniques see Route Summarisation & Aggregation:

! ── On R1: Summarise LANs toward R2 and R3 ───────────────
NetsTuts_R1(config)#interface GigabitEthernet0/1
NetsTuts_R1(config-if)#ip summary-address eigrp 100 192.168.10.0 255.255.255.0
NetsTuts_R1(config-if)#exit
NetsTuts_R1(config)#interface GigabitEthernet0/2
NetsTuts_R1(config-if)#ip summary-address eigrp 100 192.168.10.0 255.255.255.0
NetsTuts_R1(config-if)#exit

Unequal-Cost Load Balancing (Variance)

EIGRP's most powerful feature — variance allows traffic to be load-balanced across paths with different metrics, as long as the alternative paths are Feasible Successors. No other common routing protocol supports this natively:

NetsTuts_R1(config)#router eigrp 100
NetsTuts_R1(config-router)#variance 2

variance 2 means: include any Feasible Successor whose FD is within 2× the Successor's FD in load balancing. If the Successor has FD 5632 and a Feasible Successor has FD 8000, it qualifies (8000 ≤ 5632×2=11264). Traffic is distributed proportionally — lower FD links carry more traffic.

Default Route in EIGRP

! ── Method 1: Redistribute a static default route ────────
NetsTuts_R1(config)#ip route 0.0.0.0 0.0.0.0 203.0.113.1
NetsTuts_R1(config)#router eigrp 100
NetsTuts_R1(config-router)#redistribute static

! ── Method 2: Network statement for 0.0.0.0 ─────────────
NetsTuts_R1(config)#ip route 0.0.0.0 0.0.0.0 203.0.113.1
NetsTuts_R1(config-router)#network 0.0.0.0

Method 1 (redistribute static) injects the default route as an External EIGRP route (AD 170, marked D EX). Method 2 (network 0.0.0.0) advertises it as an Internal EIGRP route (AD 90). In both cases, all EIGRP neighbors automatically learn the default route. For default route concepts see Default Routes.

7. Verification

show ip eigrp neighbors

NetsTuts_R1#show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq
                                                   (sec)         (ms)       Cnt Num
1   10.0.13.2               Gi0/2                    13 00:08:12    5   200  0  12
0   10.0.12.2               Gi0/1                    14 00:08:45    4   200  0  15

Two neighbors confirmed — R2 (10.0.12.2) and R3 (10.0.13.2) — both on GigabitEthernet interfaces. Hold timer counts down from 15 seconds (3×Hello) and resets each time a Hello is received. Q Cnt = 0 means no packets are queued for transmission — a healthy sign.

show ip eigrp neighbors — Field Reference

Field	Meaning	Concern if...
H	Handle — order in which neighbor was discovered	—
Hold	Remaining time before neighbor declared down (resets on each Hello)	If frequently near 0 — Hello packets being delayed or dropped
SRTT	Smooth Round-Trip Time in ms — used to calculate retransmission timeout	High values (>5000ms) indicate a poor link
RTO	Retransmission Timeout — how long EIGRP waits before retransmitting	High values indicate link quality issues
Q Cnt	Queue count — packets waiting to be sent to this neighbor	Consistently non-zero indicates congestion or a stuck adjacency

show ip eigrp topology

NetsTuts_R1#show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(100)/ID(1.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 192.168.30.0/24, 1 successors, FD is 5632
        via 10.0.13.2 (5632/2816), GigabitEthernet0/2
        via 10.0.12.2 (8448/5632), GigabitEthernet0/1

P 192.168.20.0/24, 1 successors, FD is 5632
        via 10.0.12.2 (5632/2816), GigabitEthernet0/1
        via 10.0.13.2 (8448/5632), GigabitEthernet0/2

P 10.0.23.0/30, 1 successors, FD is 5632
        via 10.0.12.2 (5632/2816), GigabitEthernet0/1
        via 10.0.13.2 (5632/2816), GigabitEthernet0/2

Reading the topology table for 192.168.30.0/24: P = Passive (stable). 1 successors. FD is 5632 — the best total cost from R1. First entry: via 10.0.13.2 with (FD=5632 / RD=2816) — this is the Successor (lowest FD). Second entry: via 10.0.12.2 with (FD=8448 / RD=5632) — the RD of 5632 is NOT less than the Successor's FD of 5632, so this does not qualify as a Feasible Successor. Only the Successor is installed in the routing table.

show ip route eigrp

NetsTuts_R1#show ip route eigrp
Codes: D - EIGRP, EX - EIGRP external

D     10.0.23.0/30 [90/5632] via 10.0.12.2, 00:08:45, GigabitEthernet0/1
                   [90/5632] via 10.0.13.2, 00:08:12, GigabitEthernet0/2
D     192.168.20.0/24 [90/5632] via 10.0.12.2, 00:08:45, GigabitEthernet0/1
D     192.168.30.0/24 [90/5632] via 10.0.13.2, 00:08:12, GigabitEthernet0/2

EIGRP routes are marked D with AD 90. The 10.0.23.0/30 link shows two equal-cost paths (ECMP load balancing — both have metric 5632). 192.168.20.0/24 and 192.168.30.0/24 each have one best path (Successor) installed.

show ip eigrp topology all-links

NetsTuts_R1#show ip eigrp topology all-links
...
P 192.168.30.0/24, 1 successors, FD is 5632, serno 7
        via 10.0.13.2 (5632/2816), GigabitEthernet0/2     <-- Successor
        via 10.0.12.2 (8448/5632), GigabitEthernet0/1     <-- Not FS (RD not < FD)

show ip eigrp topology all-links shows all known paths — including those that do not qualify as Feasible Successors. The default show ip eigrp topology only shows Successors and Feasible Successors. Use all-links when troubleshooting why a backup path is not being used.

Verification Command Summary

Command	What It Shows	Primary Use
`show ip eigrp neighbors`	All EIGRP neighbors — Hold timer, SRTT, Q count	First check — confirm adjacency is established
`show ip eigrp topology`	Topology table — Successor, Feasible Successor, FD and RD per route	Verify DUAL results and backup path availability
`show ip eigrp topology all-links`	All known paths including non-FS candidates — shows why a path is not a FS	Troubleshoot missing Feasible Successor
`show ip route eigrp`	Only EIGRP-installed routes — D (internal) and D EX (external)	Confirm routes are in the routing table
`show ip eigrp interfaces`	Interfaces participating in EIGRP — peer count, Hello interval	Confirm correct interfaces are in EIGRP
`show ip route` / `show ip route eigrp`	Full routing table — EIGRP routes as D, EIGRP external as D EX	Final end-to-end route check

8. Troubleshooting EIGRP Issues

Problem	Symptom	Cause	Fix
No EIGRP neighbor forms	`show ip eigrp neighbors` is empty	AS number mismatch — one router uses AS 100, the other uses AS 200	Verify AS numbers match on both routers: `show ip protocols` — confirm "Routing for Networks" and AS number. See also Troubleshooting Layer 3 Routing
Neighbor forms then drops repeatedly	NBRCHANGE messages cycling up/down	Hello or Hold timer mismatch — or K value mismatch. Both prevent stable adjacency	Check `show ip eigrp interfaces detail` — compare Hello/Hold timers on both sides. Verify K values match with `show ip protocols`. Check physical layer with `show interfaces`
Network not in routing table	Neighbors are up but a specific network is missing from `show ip route`	No `network` statement for that interface on the remote router — it is not being advertised	Run `show ip eigrp topology` — if the network is absent, the advertising router is not including it. Check its `network` statements
Route Active — Stuck In Active (SIA)	Route stays in Active (A) state in topology table for more than 3 minutes	A neighbor sent a Query but never received a Reply — typically due to a failed link further in the network or an overloaded router	Check connectivity to all routers in the path. Look for routers with high CPU. SIA causes the adjacency to be reset — check NBRCHANGE logs
auto-summary causing black holes	Some subnets unreachable despite EIGRP routes present	`auto-summary` is enabled — EIGRP is summarizing to classful boundaries, hiding specific subnets	Add `no auto-summary` under `router eigrp [AS]` on all routers. Always disable this in modern networks.
No Feasible Successor available	When primary link fails, convergence takes several seconds instead of instant	The backup path's RD is not less than the Successor's FD — it failed the Feasibility Condition check	View `show ip eigrp topology all-links` — compare RD of backup path vs FD of Successor. Adjust interface delay (`ip delay`) to influence metric and satisfy the Feasibility Condition

Key Points & Exam Tips

EIGRP's AS number must match between neighbors — unlike OSPF's process ID which is locally significant. Mismatched AS numbers prevent adjacency.
Always configure no auto-summary under EIGRP — the default in older IOS auto-summarises to classful boundaries, causing black holes in networks with discontiguous subnets.
EIGRP's metric is a composite of bandwidth and delay by default (K1=1, K3=1). K values must match between neighbors or adjacency fails.
Feasible Distance (FD) = total cost from this router to destination. Reported Distance (RD) = cost as reported by the neighbor (from the neighbor to the destination).
A Successor is the neighbor with the lowest FD — installed in the routing table. A Feasible Successor is a backup neighbor whose RD is strictly less than the Successor's FD.
When a Feasible Successor exists, EIGRP switches to it instantly (milliseconds) when the Successor fails — no queries, no recalculation. This is EIGRP's key convergence advantage.
A route in Passive (P) state is stable. A route in Active (A) state is undergoing DUAL recalculation — a stuck Active route (SIA) indicates a serious network problem.
EIGRP supports unequal-cost load balancing via the variance command — no other common IGP does this. The variance multiplier defines how much worse than the best path an FS can be and still be included.
EIGRP routes appear as D in the routing table (AD 90). External EIGRP routes (redistributed) appear as D EX (AD 170).
show ip eigrp topology shows only Successors and Feasible Successors. Use show ip eigrp topology all-links to see all paths including those that failed the Feasibility Condition.

Next Steps: With EIGRP providing fast-converging dynamic routing, continue to DHCP Server Configuration to dynamically assign IP addresses to hosts across EIGRP-routed networks. For comparing routing protocol selection in a mixed environment, revisit OSPF Single-Area Configuration and OSPF Multi-Area Configuration. For route summarisation at scale see Route Summarisation & Aggregation. For EIGRP running over DMVPN overlays see DMVPN Phase 1, 2 & 3. For troubleshooting Layer 3 routing issues see Troubleshooting Layer 3 Routing and Troubleshooting OSPF Neighbour Adjacency.

TEST WHAT YOU LEARNED

1. R1 uses `router eigrp 100` and R2 uses `router eigrp 200`. Both are connected on the same subnet. What happens?

A. They form a neighbor relationship but cannot exchange routes B. They form a neighbor relationship — AS numbers are locally significant like OSPF process IDs C. IOS automatically negotiates a common AS number between them D. No neighbor relationship forms — EIGRP AS numbers must match on both routers. The AS number is carried in Hello packets and checked before adjacency is established

Correct answer is D. Unlike OSPF's process ID (which is locally significant and never carried in protocol messages), the EIGRP AS number is included in Hello packets and is a mandatory matching parameter. If R1 sends Hellos with AS 100 and R2 expects AS 200, R2 discards R1's Hellos. No adjacency forms. This is a fundamental difference between EIGRP and OSPF configuration — always verify AS numbers match when troubleshooting EIGRP neighbor issues.

2. R1's topology table shows: 192.168.30.0/24 via R3 (FD=5000/RD=2000) as Successor, and via R2 (FD=7000/RD=4500). Does R2 qualify as a Feasible Successor?

A. Yes — R2's FD of 7000 is higher than the Successor's FD, which is the requirement for a Feasible Successor B. No — R2's RD of 4500 is not strictly less than the Successor's FD of 5000. The Feasibility Condition requires RD < Successor's FD. 4500 < 5000 is true — wait, actually YES it qualifies C. No — only one Feasible Successor is allowed per route in EIGRP D. Yes — R2's RD of 4500 is strictly less than the Successor's FD of 5000, satisfying the Feasibility Condition. R2 qualifies as a Feasible Successor and is stored in the topology table as an instant-failover backup

Correct answer is D. The Feasibility Condition: a neighbor qualifies as a Feasible Successor if its Reported Distance (RD) is strictly less than the current Successor's Feasible Distance (FD). Here: R2's RD = 4500, Successor's FD = 5000. Since 4500 < 5000, R2 satisfies the Feasibility Condition and qualifies as a Feasible Successor. If R3 (the Successor) fails, R1 instantly promotes R2 to Successor without sending any Query packets.

3. Why must `no auto-summary` always be configured in modern EIGRP deployments?

A. Auto-summary summarises routes to classful boundaries (e.g., 10.1.1.0/24 becomes 10.0.0.0/8). In discontiguous networks where the same major network exists on both sides of a router, this creates routing black holes as routers advertise overlapping summaries pointing in wrong directions B. Auto-summary causes EIGRP to use OSPF-style area summarization instead of interface-level summarization C. Auto-summary prevents Feasible Successors from being calculated correctly D. Auto-summary increases convergence time by forcing a full DUAL recalculation every 30 seconds

Correct answer is A. Auto-summary automatically summarizes subnets to their classful network boundary at major network boundaries. For example, if R1 has 10.1.1.0/24 and R2 has 10.2.2.0/24, auto-summary causes both to advertise 10.0.0.0/8 — creating ambiguity. In discontiguous networks (same major network on different sides of a different major network), this causes black holes as both routers think they're the best path for all of 10.0.0.0/8. Always disable with no auto-summary.

4. A route shows state A (Active) in `show ip eigrp topology` for several minutes. What does this indicate?

A. The route is actively forwarding traffic — A means the route is in use B. The route has been manually set to Active to bypass DUAL C. DUAL is recalculating this route — Query packets have been sent to neighbors awaiting Replies. If stuck Active for more than ~3 minutes, it becomes SIA (Stuck In Active), causing the adjacency to reset D. The route is in active state because it has two equal-cost paths currently being load-balanced

Correct answer is C. In EIGRP, routes are normally in Passive (P) state — stable with a known Successor. When the Successor fails and no Feasible Successor exists, DUAL initiates a diffusing computation — the route enters Active (A) state and the router sends Query packets to all EIGRP neighbors. The route stays Active until all Replies are received. If a neighbor does not reply within ~3 minutes (SIA timer), the router resets that adjacency — this is Stuck In Active (SIA), a serious EIGRP problem.

5. How does EIGRP's unequal-cost load balancing using `variance 2` work?

A. It splits traffic equally between any two paths regardless of their metric B. It uses two separate routing tables — one for primary and one for backup paths C. Variance 2 doubles the AD of all EIGRP routes to reduce preference over static routes D. Any Feasible Successor whose FD is within 2× the Successor's FD is included in load balancing. Traffic is distributed proportionally — the better (lower FD) path carries more traffic than the worse path

Correct answer is D. The variance [multiplier] command enables unequal-cost load balancing. A Feasible Successor qualifies for load balancing if its FD ≤ Successor's FD × variance. With variance 2 and Successor FD = 5000, any FS with FD ≤ 10000 is included. Traffic is distributed proportionally to the inverse of the metric — a path with FD 5000 carries twice the traffic of a path with FD 10000. Critically, only Feasible Successors (those satisfying the Feasibility Condition) can participate — this ensures loop-free load balancing.

6. What does a consistently non-zero Q Cnt value in `show ip eigrp neighbors` indicate?

A. The neighbor has more routes than normal — Q Cnt tracks the route count B. Packets are queued and not being sent to the neighbor — this can indicate link congestion, a stuck adjacency, or the neighbor is not acknowledging EIGRP reliable packets C. The neighbor is running a different EIGRP AS number — the Queue is holding packets pending AS negotiation D. Q Cnt shows the number of routes being load-balanced — a higher number means more ECMP paths

Correct answer is B. Q Cnt (Queue Count) shows the number of EIGRP packets waiting to be sent to that neighbor. A healthy neighbor shows Q Cnt = 0. If Q Cnt is persistently non-zero, it means EIGRP reliable packets (Updates, Queries, or Replies) are not being acknowledged by the neighbor. This can indicate link congestion causing packet drops, an overloaded neighbor router, or a failing adjacency. Investigate with show interfaces [int] to check for errors and drops.

7. What is the Administrative Distance of an EIGRP external route (D EX) and when does it appear?

A. AD 170 — it appears when routes from another routing protocol, a static route, or a connected route are redistributed into the EIGRP domain. It is less trusted than internal EIGRP (AD 90) to prevent routing loops B. AD 90 — same as internal EIGRP, but marked EX for informational purposes only C. AD 110 — same as OSPF, since external routes are treated identically regardless of which protocol learned them D. AD 170 — it appears when EIGRP receives routes from another EIGRP AS with a different AS number

Correct answer is A. EIGRP has two Administrative Distances: Internal (AD 90) for routes learned within the same EIGRP AS, and External (AD 170) for routes that were redistributed from an external source (static routes, other routing protocols, or connected routes) into EIGRP. The higher AD for external routes (170 vs 90) means a router would prefer an internal EIGRP route over an external one to the same destination — reducing the risk of routing loops through redistribution points.

8. What is the key difference between `show ip eigrp topology` and `show ip eigrp topology all-links`?

A. The default command shows all paths; all-links filters to only the best path B. all-links includes routes from other routing protocols redistributed into EIGRP C. The default command shows only Successors and Feasible Successors. all-links shows every known path including those that failed the Feasibility Condition — useful for understanding why a backup path is not a Feasible Successor D. all-links includes passive interfaces that are not actively participating in EIGRP

Correct answer is C. show ip eigrp topology displays only loop-free paths — Successors and Feasible Successors. Paths that failed the Feasibility Condition (their RD is not less than the Successor's FD) are hidden. show ip eigrp topology all-links reveals these hidden paths — showing their FD and RD values so you can see exactly why they did not qualify. This is the essential command when troubleshooting why no instant-failover backup exists for a route.

9. EIGRP Hello interval on Ethernet is 5 seconds. What is the default Hold timer and what happens when it expires?

A. Hold timer is 10 seconds — the router sends a BYE packet and gracefully removes the neighbor B. Hold timer is 20 seconds — the route enters Active state immediately on expiry C. Hold timer is 15 seconds — the router retransmits Hello packets 3 times before marking the neighbor down D. Hold timer is 15 seconds (3 × Hello interval). If no Hello is received within 15 seconds, the neighbor is declared down, its routes are removed from the topology table, and DUAL begins recalculation for affected routes

Correct answer is D. EIGRP's default Hold timer on Ethernet is 15 seconds — three times the Hello interval of 5 seconds. The Hold timer counts down from 15 and resets each time a Hello packet is received from that neighbor. If it reaches zero (no Hello for 15 seconds), the neighbor is immediately declared down. All routes learned from that neighbor are removed from the topology table, and DUAL recalculates affected routes — either promoting a Feasible Successor instantly or entering Active state to find a new path.

10. R1 has a Feasible Successor for 192.168.30.0/24. R1's link to the Successor goes down. What is EIGRP's immediate response?

A. EIGRP sends Query packets to all neighbors and waits for Replies before switching to the backup path B. The Feasible Successor is instantly promoted to Successor with no Query packets sent — the route stays in Passive state throughout. Traffic switches to the backup path in milliseconds C. The route enters Active state while EIGRP verifies the Feasible Successor is still valid D. EIGRP waits for the Hold timer to expire before checking for backup paths

Correct answer is B. This is EIGRP's defining feature — when a Feasible Successor exists, failover is instant. Because the Feasibility Condition mathematically guarantees the backup path is loop-free, EIGRP does not need to query anyone. The FS is immediately promoted to Successor, the routing table is updated, and traffic flows via the new path — all within milliseconds. The route remains in Passive (P) state throughout. This instant failover only works when a pre-validated Feasible Successor exists in the topology table.

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