Cisco 300-410: EIGRP Design, Metrics and Convergence — Study Guide

By Laurynas Brazys · 16 August 2026 · #cisco #300-410 #certification #exam-prep #study-guide

Part of the Cisco CCNP Enterprise 300-410 ENARSI — Study Guide. Practice with verified answers in the Cisco exam hub, or take timed practice tests on ExamRoll.io.

Overview

Enhanced Interior Gateway Routing Protocol (EIGRP) is a fast-converging, loop-free, distance-vector routing protocol that uses the Diffusing Update Algorithm (DUAL) to compute backup paths and avoid transient loops. Design choices around metrics, path selection, query containment, summarization, neighbor formation, authentication, and redistribution directly influence stability and convergence time. This section describes how to design, configure, and troubleshoot EIGRP for deterministic behavior across IPv4 and IPv6 deployments.

DUAL Operation and Path Selection

EIGRP uses DUAL to maintain a loop-free topology and accelerate convergence.

• Successor: The primary next hop for a destination. Installed in the routing table.
• Feasible Distance (FD): The best known metric from the local router to a destination (via the successor).
• Reported Distance (RD): The metric to the destination reported by a neighbor (also called advertised distance).
• Feasible Successor (FS): A backup next hop that is guaranteed loop-free by the Feasibility Condition (FC).

Feasibility Condition: A neighbor qualifies as a feasible successor if its RD to the destination is strictly less than the local FD to that destination via the current successor: RDneighbor < FDlocal. This guarantees that the neighbor is closer to the destination than the local router, preventing loops without requiring a full SPF computation.

Behavioral outcomes:

• If a destination loses its successor and at least one FS exists, the router performs an immediate local switchover with no queries, yielding subsecond convergence on LAN-speed links.
• If no FS exists, the destination goes Active and the router sends queries to neighbors to find a replacement. Query scoping design becomes critical to avoid delays.

Unequal-cost load balancing with variance:

• EIGRP installs multiple paths when variance is configured and those paths are FSs. A path is eligible if its FD ≤ (variance × FD of best successor). Only FSs can be installed for traffic sharing; equal-cost non-FS paths are not used to avoid loops.
• Traffic sharing can be balanced (default, proportional to inverse metrics) or minimized with traffic-share min across-interfaces.

Example: router eigrp 100 variance 2 traffic-share balanced

Design note: If multiple links exist but do not meet the FC, consider adjusting interface delay (not bandwidth) to influence FD/RD relationships. Do not modify K values for this purpose.

Configuration Models, Neighbor Formation, and Authentication

EIGRP supports classic and named configuration models.

Classic EIGRP (IPv4): router eigrp 100 network 10.0.0.0 0.0.255.255 passive-interface GigabitEthernet0/2

• Per-interface summarization and authentication are configured at the interface.

Named EIGRP (consolidates IPv4/IPv6 and centralizes policy): router eigrp FABRIC address-family ipv4 unicast autonomous-system 100 af-interface default passive-interface network 10.0.0.0 0.0.255.255 address-family ipv6 unicast autonomous-system 100 af-interface GigabitEthernet0/1 no shutdown

Neighbor discovery:

• Hello/hold timers: default 5/15 seconds on high-speed links, 60/180 on low-speed. Timers need not match; the hold time accepted is what the neighbor advertises.
• Multicast destinations: 224.0.0.10 (IPv4) and FF02::A (IPv6).
• Passive-interface suppresses hellos; use on access-facing ports or where no adjacency should form.

Authentication:

• Classic per-interface MD5/HMAC-SHA authentication: interface GigabitEthernet0/0 ip authentication mode eigrp 100 md5 ip authentication key-chain eigrp 100 EIGRP-KEYS

• In named mode, apply authentication under af-interface. All neighbors on a segment must share algorithm and keys; mismatches prevent adjacency.

Stub routing:

• Configure only on the stub itself; neighbors learn the stub capability and suppress nonessential queries. router eigrp 100 eigrp stub connected summary

Default stub options advertise connected and summary routes. Add static or redistributed as needed.

IPv6 EIGRP:

• Requires a 32-bit router ID and per-interface activation. ipv6 unicast-routing ipv6 router eigrp 100 eigrp router-id 1.1.1.1 ! interface GigabitEthernet0/1 ipv6 address 2001:db8:1::1/64 ipv6 eigrp 100

Metrics: Composite and Wide, K Values, and Compatibility

Composite metric (classic):

• Default K values: K1=1 (bandwidth), K3=1 (delay), K2=K4=K5=0. Effective metric = 256 × (min link bandwidth inverse + cumulative delay). Load and reliability are ignored by default.
• Do not change K values in production designs; all neighbors must match K values exactly, or adjacencies fail.

Wide metrics:

• Wide metrics extend metric scale and precision (notably for very high bandwidth/low delay links) and add headroom for TE features. All neighbors must support and negotiate the same EIGRP metric version. Mismatched metric versions or K values prevent adjacency.
• Metric manipulation best practices:
  • Prefer changing interface delay to influence path preference; it is additive and deterministic.
  • Avoid setting unrealistic bandwidth; it also affects QoS and other subsystems.
  • Use default-metric or explicit metrics for redistribution to ensure predictable path selection.

Redistribution metrics: router eigrp 100 default-metric 100000 10 255 1 1500 redistribute ospf 1 route-map TAG-OSPF

Convergence, Queries, Summarization, and Query Boundaries

Queries and Stuck-in-Active (SIA):

• When no FS exists, routes go Active and the router sends queries to all neighbors except those that are stubs or have summarization boundaries. Each queried neighbor must reply before the active-time expires (default 3 minutes). Failure to receive all replies causes SIA; the neighbor may be reset and the route purged.
• EIGRP SIA-Query/SIA-Reply enhancements detect slow responders earlier, but design is the primary mitigation.

Query containment strategies:

• Summarization: Create aggregates at distribution or ABR-like boundaries to prevent queries from crossing. EIGRP installs a local Null0 discard route for the summary (admin distance 5) with metric equal to the best component route. This both reduces query scope and protects against blackholing when specific routes are missing.
• Stub routing: Configure at spokes or leaf sites to stop transit queries through limited devices.
• Filtering: Limit propagation of unneeded routes and reduce the topology footprint.

IPv4 summarization with leak-map: interface GigabitEthernet0/0 ip summary-address eigrp 100 10.16.0.0 255.255.0.0 leak-map EIGRP-LEAK ! ip prefix-list CRITICAL permit 10.16.200.0/24 route-map EIGRP-LEAK permit 10 match ip address prefix-list CRITICAL

Leak-maps allow advertising selected more-specifics alongside the summary—for example, to steer traffic for critical subnets via policy or to maintain optimal paths while still containing queries for the rest.

Design trade-offs:

• Coarse summaries maximize stability but may obscure suboptimal paths, causing traffic to follow longer routes. Leak only what is necessary.
• Excessive variance can increase load balancing paths but only among FSs; ensure query containment so that FSs exist for critical destinations.

Policy, Redistribution, Verification, and Troubleshooting

Redistribution into/out of EIGRP:

• Routes redistributed into EIGRP become external (admin distance 170). Always set a deterministic metric and apply tagging to prevent loops on mutual redistribution. router eigrp 100 redistribute ospf 1 metric 100000 10 255 1 1500 route-map SET-TAG ! route-map SET-TAG permit 10 set tag 65001 ! router ospf 1 redistribute eigrp 100 subnets route-map BLOCK-RETAGS ! route-map BLOCK-RETAGS deny 10 match tag 65001 route-map BLOCK-RETAGS permit 20

Verification essentials:

• Neighbors: show ip eigrp neighbors detail, show ipv6 eigrp neighbors
• Topology: show ip eigrp topology all-links, show ip eigrp topology prefix, and named-mode equivalents under show eigrp address-family
• Routing: show ip route eigrp, show ipv6 route eigrp
• Protocol state: show ip protocols, show eigrp address-family ipv4 events
• Traffic and queries: show ip eigrp traffic

Convergence troubleshooting workflow:

1. Confirm adjacency prerequisites: matching AS, K values/metric version, authentication, and no passive-interface on transit links.
2. Inspect timers and interface health; flaps cause frequent actives. Adjust hello/hold only when necessary; prefer fixing underlying media issues.
3. Check for SIA indications and query storms. Add or refine summaries and configure stub on leaf routers to bound queries.
4. Evaluate FS availability in the topology. If missing, verify that feasible condition can be met; adjust delays to create viable backups if design requires.
5. Validate redistribution metrics and tags. Missing defaults yield infinite metrics, dropping routes; absent tags can create loops.
6. For IPv6, ensure router ID is set and per-interface activation is present; IPv6 EIGRP does not use IPv4 network statements.

Practical Problem Scenario

Northwind Logistics operates a dual-hub EIGRP network with dozens of spoke warehouses. Occasional access-circuit flaps at remote sites trigger query storms, causing intermittent SIA on the hubs and delayed failovers. The company also plans to enable IPv6 alongside IPv4 and must prevent mutual redistribution loops between EIGRP and OSPF at regional data centers.

Approach:

1. Bound queries with summarization at the distribution layer.

• On each distribution interface toward spokes, configure per-interface IPv4 summaries and leak critical subnets that require optimal routing. This reduces the query scope when a spoke loses a more-specific while preserving performance for key prefixes. interface GigabitEthernet2/0 ip summary-address eigrp 100 10.64.0.0 255.255.0.0 leak-map CRIT-LEAK ! ip prefix-list CRIT permit 10.64.10.0/24 route-map CRIT-LEAK permit 10 match ip address prefix-list CRIT Rationale: Summaries create a Null0 discard for unmatched subprefixes (AD 5) and prevent Active states from propagating past the boundary, sharply reducing SIA risk.

2. Declare spokes as stubs with only connected and summary routes. router eigrp 100 eigrp stub connected summary Rationale: Hubs will not send wide-ranging queries to spokes; spokes need not respond to routes they cannot improve, shortening convergence and conserving CPU/memory on low-end CPE.

3. Enable unequal-cost load sharing between dual hubs where FC is satisfied. router eigrp 100 variance 2 traffic-share balanced Rationale: Variance allows using multiple FS paths to hubs, improving throughput and resilience without violating loop-free guarantees, provided FC holds.

4. Standardize metrics and avoid K value changes.

• Do not alter K values. Set redistribution metrics explicitly at data centers. router eigrp 100 default-metric 100000 10 255 1 1500 redistribute ospf 10 route-map TAG-INTO-EIGRP route-map TAG-INTO-EIGRP permit 10 set tag 65010 Rationale: Consistent metrics yield predictable path selection; tags mark external routes to prevent re-entry loops.

5. Block redistribution loops from EIGRP back into OSPF. router ospf 10 redistribute eigrp 100 subnets route-map BLOCK-TAG route-map BLOCK-TAG deny 10 match tag 65010 route-map BLOCK-TAG permit 20 Rationale: Tags prevent the same routes from oscillating between protocols, avoiding churn and metric confusion.

6. Harden neighbor formation with authentication on hub LAN segments. interface Port-Channel10 ip authentication mode eigrp 100 md5 ip authentication key-chain eigrp 100 NW-KEYS Rationale: Prevents unauthorized adjacencies and accidental metric/K-value mismatches from third-party devices.

7. Deploy IPv6 EIGRP per interface and set a router ID. ipv6 unicast-routing ipv6 router eigrp 100 eigrp router-id 10.255.255.1 ! interface GigabitEthernet2/0 ipv6 address 2001:db8:64::1/64 ipv6 eigrp 100 Rationale: IPv6 EIGRP requires explicit per-interface enablement and a 32-bit router ID; this mirrors IPv4 behavior with separate adjacencies over FF02::A.

8. Validate and monitor.

• Use show ip eigrp topology all-links to verify FS presence; show ip eigrp neighbors detail to confirm timers/authentication; show ip eigrp traffic to ensure query counts drop post-change. Rationale: Confirms that the design changes reduce actives/SIA and that multiple FSs are available for rapid switchover.

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