8.2 Enhanced Interior Gateway Routing Protocol (EIGRP)

Key Takeaways

EIGRP is an advanced distance-vector routing protocol that uses the Diffusing Update Algorithm (DUAL) to guarantee 100% loop-free topology calculations and achieve sub-second convergence without full-table periodic flooding.
The Feasibility Condition (FC) is the cornerstone of EIGRP loop prevention: a neighbor is accepted as a Feasible Successor if and only if its Reported Distance is strictly less than the current Successor's Feasible Distance (RD < FD_successor).
Classic EIGRP metrics use a 32-bit formula based on Minimum Bandwidth and Cumulative Delay (default K1=1, K3=1), whereas modern Named Mode implements 64-bit Wide Metrics with picosecond latency scaling to accommodate high-speed links (>10 Gbps).
EIGRP Named Mode unifies IPv4, IPv6, VRFs, and interface parameters under a structured hierarchical CLI architecture (router eigrp NAME -> address-family -> af-interface -> topology base).
Stuck-in-Active (SIA) conditions are mitigated using SIA-Query and SIA-Reply handshakes initiated at 50% of the active timer (90 seconds), while EIGRP Stub routing restricts query propagation to hub-and-spoke branch edges by default (connected summary).

Last updated: August 2026

8.2 Enhanced Interior Gateway Routing Protocol (EIGRP)

The Enhanced Interior Gateway Routing Protocol (EIGRP) is an enterprise-class, advanced distance-vector routing protocol developed by Cisco (and published in informational RFC 7868). EIGRP combines the operational simplicity and low resource consumption of distance-vector protocols with the rapid convergence, loop-free mathematical guarantees, and hierarchical scalability of link-state protocols.

+-----------------------------------------------------------------------------------+
|                         EIGRP PROTOCOL CHARACTERISTICS                            |
+-----------------------------------------------------------------------------------+
|  - Transport Protocol:      Reliable Transport Protocol (RTP), IP Protocol 88     |
|  - Multicast Addresses:     224.0.0.10 (IPv4) / FF02::A (IPv6)                    |
|  - Algorithm:               Diffusing Update Algorithm (DUAL - RFC 7868)          |
|  - Convergence Mechanism:   Pre-computed Feasible Successor (Instant 0ms Failover)|
|  - Update Behavior:         Non-periodic, partial, bounded triggered updates      |
|  - Metric Architecture:     Composite (Bandwidth + Delay); Classic 32b / Wide 64b |
|  - Multi-Pathing:           ECMP (Equal-Cost) & UCMP (Unequal-Cost via Variance)  |
|  - Configuration Models:    Classic Mode (Autonomous System) vs. Named Mode       |
+-----------------------------------------------------------------------------------+

1. DUAL Finite State Machine & Loop-Free Terminology

At the core of EIGRP lies the Diffusing Update Algorithm (DUAL), developed by Dr. J.J. Garcia-Luna-Aceves. DUAL models the network as a finite state machine that guarantees loop freedom at every instant during topological recalculation, eliminating the counting-to-infinity and routing loop issues inherent in legacy distance-vector protocols (such as RIP).

+-----------------------------------------------------------------------------------+
|                      EIGRP TOPOLOGY & DUAL TERMINOLOGY                            |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|                                [ Router A ]                                       |
|                               /            \                                      |
|           Path 1: Metric 100 /              \ Path 2: Metric 150                  |
|                             v                v                                    |
|                       [ Router B ]      [ Router C ]                              |
|                             \                /                                    |
|            Path 1: Metric 50 \              / Path 2: Metric 60                   |
|                               v            v                                      |
|                            [ Destination 10.1.1.0/24 ]                            |
|                                                                                   |
|  From Router A's perspective:                                                     |
|  - Router B Reported Distance (RD) = 50                                           |
|  - Router B Feasible Distance (FD) = 100 + 50 = 150  <-- LOWEST METRIC (SUCCESSOR)|
|  - Router C Reported Distance (RD) = 60                                           |
|  - Router C Computed Total Distance = 150 + 60 = 210                              |
|  - Feasibility Condition Check for Router C: RD (60) < FD_successor (150) -> PASS!|
|  - Router C is a FEASIBLE SUCCESSOR (Backup next-hop)                             |
+-----------------------------------------------------------------------------------+

Core DUAL Definitions

Reported Distance (RD) / Advertised Distance (AD):
- The metric to a destination prefix as calculated and advertised by an immediate neighboring router.
- It represents the neighbor's own total Feasible Distance from itself to the target network.
Feasible Distance (FD):
- The lowest calculated metric to a destination prefix from the local router across all paths since the route last transitioned from the Active state to the Passive state.
- The path providing this lowest metric becomes the primary forwarding route.
Successor:
- The neighboring next-hop router that possesses the lowest metric path (Feasible Distance) to the destination.
- The Successor route is authoritatively installed into the Routing Information Base (RIB) and programmed into the hardware CEF FIB.
Feasible Successor (FS):
- A backup next-hop router that provides an alternate, mathematically proven loop-free path to the destination.
- The Feasible Successor is maintained in the EIGRP Topology Table (show ip eigrp topology). If the primary Successor link fails, the FS is instantly promoted to Successor in hardware with zero convergence delay (0ms) and without sending query packets.
The Feasibility Condition (FC):
- The universal mathematical rule governing loop-free backup path validation: $\mathbf{RD}_{\text{Candidate}} < \mathbf{FD}_{\text{Current Successor}}$
- Why it guarantees loop freedom: If a neighboring router reports a metric to the destination that is strictly less than our own lowest historical cost (FD), that neighbor cannot possibly be routing packets back through us. If the neighbor were routing through us, its reported distance would have to include our cost, making its RD greater than our FD.
Passive (P) vs. Active (A) State:
- Passive (P): The normal, healthy operational state for a route. The route is stable, traffic is forwarding, and DUAL is not performing recalculations.
- Active (A): The route has lost its primary Successor, has no valid Feasible Successor in the topology table, and DUAL has actively initiated the Query process to locate an alternative path across neighboring routers. Packets destined for an Active route may be dropped or delayed during recalculation.

Loading diagram...

DUAL State Transition and Feasible Successor Promotion Workflow

2. EIGRP Metric Calculations: Classic Metrics vs. 64-Bit Wide Metrics

EIGRP utilizes a composite metric structure based on network path characteristics. The metric is computed using weighted constants known as K-values ($K1$ through $K5$).

The Five K-Values and Defaults

$K1$ = Bandwidth Weight (Default: 1)
$K2$ = Load Weight (Default: 0)
$K3$ = Delay Weight (Default: 1)
$K4$ = Reliability Weight (Default: 0)
$K5$ = Reliability Factor (Default: 0)

K-Value Adjacency Requirement: K-values are transmitted inside EIGRP Hello packets. For two routers to establish an EIGRP neighbor adjacency, their K-values must match identically. If a K-value mismatch occurs, the routers will log continuous K-value mismatch syslog warnings and refuse to form an adjacency.

1. Classic EIGRP Composite Metric Formula (32-Bit)

Under default K-value settings ($K1=1, K2=0, K3=1, K4=0, K5=0$), the full mathematical formula collapses to a simple composite of Minimum Bandwidth and Cumulative Delay:

$\text{Full Formula: } \text{Metric} = 256 \times \left[ \left( K1 \cdot \text{BW}_{\min} + \frac{K2 \cdot \text{BW}_{\min}}{256 - \text{Load}} + K3 \cdot \text{Delay}_{\text{sum}} \right) \times \left( \frac{K5}{\text{Reliability} + K4} \right) \right]$

$\mathbf{\text{Default Classic Metric}} = 256 \times \left( \frac{10^7}{\text{Bandwidth}_{\min\text{ (kbps)}}} + \frac{\text{Cumulative Delay (}\mu\text{s)}}{10} \right)$

Bandwidth Component: $\frac{10^7}{\text{Slowest Link Bandwidth in kbps}}$. This scales inversely: higher bandwidth yields a lower metric cost.
Delay Component: Cumulative sum of interface delays along the entire path (expressed in units of tens of microseconds, $\mu\text{s} / 10$).

! Step-by-Step Classic Metric Calculation Example:
Path traverses two GigabitEthernet links (1,000,000 kbps, 10 microseconds delay each):
1. Minimum Bandwidth = 1,000,000 kbps -> 10,000,000 / 1,000,000 = 10
2. Cumulative Delay = 10 us + 10 us = 20 us -> 20 / 10 = 2
3. Composite Base = 10 + 2 = 12
4. EIGRP 32-Bit Scaled Metric = 12 * 256 = 3,072

Limitations of Classic Metrics on High-Speed Interfaces

Classic EIGRP metrics were designed when $10\text{ Mbps}$ (Ethernet) and $100\text{ Mbps}$ (FastEthernet) were the standard. Because the bandwidth calculation is capped at $\frac{10^7}{\text{BW}_{\text{kbps}}}$, any interface of $10\text{ Gbps}$ ($10,000,000\text{ kbps}$) or faster results in a bandwidth cost of $1$ ($rac{10^7}{10^7} = 1$). Consequently, classic EIGRP cannot differentiate between a $10\text{ Gbps}$, $40\text{ Gbps}$, $100\text{ Gbps}$, or $400\text{ Gbps}$ interface, basing path selection entirely on interface delay.

2. 64-Bit Wide Metrics (EIGRP Named Mode)

To scale across modern high-speed campus and data center fabrics, Cisco introduced 64-bit Wide Metrics (RFC 7868), which is the default operating metric in EIGRP Named Mode. Wide metrics support interfaces up to $4.29\text{ Terabits/sec}$ and measure latency in picoseconds ($10^{-12}$ seconds) rather than tens of microseconds ($10^{-5}$ seconds).

$\mathbf{\text{Throughput}} = \frac{10^7 \times 65536}{\text{Minimum Bandwidth (kbps)}}$

$\mathbf{\text{Latency}} = \frac{\text{Cumulative Delay (picoseconds)} \times 65536}{10^6}$

$\mathbf{\text{Wide Composite Metric}} = (K1 \times \text{Throughput}) + (K3 \times \text{Latency})$

RIB Scale Factor: Because the Cisco IOS-XE Routing Information Base (RIB) only accepts 32-bit metrics, EIGRP Named Mode scales its internal 64-bit wide metric by dividing it by a default scale factor of 128 before inserting the route into the global routing table: $\text{RIB Metric} = \frac{\text{Wide Metric}}{128}$.

Metric Comparison: Classic vs. Wide Metrics

Attribute	Classic EIGRP Metric	64-Bit Wide EIGRP Metric
Bit Width	32-bit integer	64-bit integer
Delay Resolution	Tens of microseconds ($10,\mu\text{s}$)	Picoseconds ($1,\text{ps} = 10^{-12},\text{s}$)
Bandwidth Scaling Factor	$10^7 / \text{BW}_{\text{kbps}}$	$(10^7 \times 65536) / \text{BW}_{\text{kbps}}$
Maximum Distinguishable Bandwidth	$10\text{ Gbps}$	$4.29\text{ Tbps}$
Supported Configuration Mode	Classic Mode & Named Mode	Named Mode only
K-Value Extension	$K1$ through $K5$	$K1$ through $K6$ ($K6 = \text{Extended Attributes / Jitter / Energy}$)

3. EIGRP Named Mode Configuration Architecture

Historically, EIGRP was configured in Classic Mode using scattered global commands (router eigrp <AS>), separate IPv6 instances (router ipv6 eigrp <AS>), and disparate interface-level subcommands (ip hello-interval eigrp, ip authentication mode eigrp).

EIGRP Named Mode introduces a unified, modular, hierarchical CLI structure that consolidates IPv4, IPv6, VRFs, interface timers, authentication, and topology controls into a single configuration block under router eigrp <VIRTUAL-INSTANCE-NAME>.

+-----------------------------------------------------------------------------------+
|                         EIGRP NAMED MODE HIERARCHY                                |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|  router eigrp ENTERPRISE-CORE                                                     |
|   |                                                                               |
|   +---> address-family ipv4 unicast autonomous-system 100                         |
|          |                                                                        |
|          +---> af-interface default | af-interface <interface-name>               |
|          |      - Hello / Hold timers                                             |
|          |      - Authentication (SHA-256 HMAC / MD5)                             |
|          |      - Passive-interface settings                                      |
|          |      - Summary-address generation                                      |
|          |                                                                        |
|          +---> topology base                                                      |
|          |      - Variance multiplier (UCMP)                                      |
|          |      - Maximum-paths (ECMP)                                            |
|          |      - Route redistribution & filtering                                |
|          |      - Offset-lists & traffic-shares                                   |
|          |                                                                        |
|          +---> network <network-ip> <wildcard-mask>                               |
|          +---> eigrp-stub [connected] [summary] [static] [receive-only]           |
+-----------------------------------------------------------------------------------+

Production IOS-XE Named Mode Configuration

! Configure an enterprise dual-stack EIGRP Named Instance with SHA-256 Authentication
router eigrp ENTERPRISE-FABRIC
 !
 ! ---------------- IPv4 Address Family Block ----------------
 address-family ipv4 unicast autonomous-system 100
  !
  ! Interface-specific parameters (Security & Timers)
  af-interface default
   passive-interface default                    ! Security baseline: suppress hellos on all ports
  exit-af-interface
  !
  af-interface GigabitEthernet0/0/1
   no passive-interface                         ! Enable EIGRP on core-facing uplink
   hello-interval 5
   hold-time 15
   authentication mode hmac-sha-256 CiscoEnc0rKey#2026
  exit-af-interface
  !
  af-interface GigabitEthernet0/0/2
   no passive-interface
   summary-address 10.1.0.0 255.255.0.0         ! Generate outbound summary route
  exit-af-interface
  !
  ! Topology-specific tuning (Multi-Pathing & Convergence)
  topology base
   variance 2                                   ! Enable Unequal-Cost Multi-Pathing (UCMP)
   maximum-paths 8                              ! Allow up to 8 multi-path routes in RIB
  exit-af-topology
  !
  ! Network Statements (Enables EIGRP on matching interfaces)
  network 10.1.0.0 0.0.255.255
  network 192.168.12.0 0.0.0.3
  eigrp-stub connected summary                  ! Configure stub routing on branch node
 exit-address-family
 !
 ! ---------------- IPv6 Address Family Block ----------------
 address-family ipv6 unicast autonomous-system 100
  af-interface default
   passive-interface default
  exit-af-interface
  !
  af-interface GigabitEthernet0/0/1
   no passive-interface
   authentication mode hmac-sha-256 CiscoEnc0rKey#2026
  exit-af-interface
  !
  topology base
   maximum-paths 4
  exit-af-topology
 exit-address-family

Migration Command: Existing classic EIGRP configurations can be non-disruptively converted to Named Mode using the privileged EXEC command: Router# eigrp upgrade-cli <INSTANCE_NAME>. The router translates all scattered interface and global subcommands into the unified hierarchical structure automatically.

4. Active State, Query Propagation, and Stuck-in-Active (SIA)

When a router loses its primary Successor route and no Feasible Successor exists in the topology table, the route transitions from Passive (P) to Active (A). DUAL must actively query neighbors to determine if an alternate loop-free path exists.

+-----------------------------------------------------------------------------------+
|                         DUAL QUERY & REPLY PROPAGATION                            |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|  1. Successor link fails on Router A. No Feasible Successor available.            |
|  2. Router A places route in ACTIVE state; starts Active Timer (180s).            |
|  3. Router A sends DUAL Query (multicast/unicast) to Router B and Router C.       |
|  4. Router B has no alternate path -> sets route ACTIVE -> queries downstreams.   |
|  5. Router C has a local path -> responds immediately with DUAL Reply.            |
|  6. Router A CANNOT complete recalculation until ALL queried neighbors reply.     |
|                                                                                   |
+-----------------------------------------------------------------------------------+

The Stuck-in-Active (SIA) Dilemma

By default, Cisco IOS-XE enforces an Active Timer of 3 minutes (180 seconds). If a router does not receive a Reply packet from every single queried neighbor before the Active Timer expires, the route is declared Stuck-in-Active (SIA).

Legacy SIA Penalty: Historically, when a route went SIA, EIGRP assumed the non-responding neighbor was dead or corrupted. The router immediately reset the entire neighbor adjacency, tearing down all established routing sessions, flushing thousands of unrelated routes, and causing massive network-wide route flaps.
Common Root Causes of SIA:
1. High CPU utilization on a downstream transit router preventing DUAL query processing.
2. Unidirectional link failure or packet drops on congested WAN links.
3. Excessive query radius (unbounded query depth across deep enterprise networks).

Modern SIA Mitigation: SIA-Query and SIA-Reply (RFC 7868)

Modern Cisco IOS-XE implements an automated SIA mitigation mechanism that prevents unnecessary neighbor tear-downs:

At 50% of the Active Timer (90 seconds), if a neighbor has not yet responded with a standard Reply, the local router sends a specialized SIA-Query unicast packet to that specific neighbor.
The neighbor receives the SIA-Query. If the neighbor is still alive and simply waiting on downstream replies, it immediately responds with an SIA-Reply packet ("I am still alive and actively searching").
Upon receiving the SIA-Reply, the local router resets its active timer for another 90 seconds without tearing down the neighbor adjacency.
Up to 3 successive SIA-Queries can be exchanged. If the neighbor fails to respond to the SIA-Query within its allotted window, the neighbor is deemed genuinely unresponsive and the adjacency is safely terminated.

! Example Syslog message when SIA mitigation terminates an unresponsive neighbor:
%DUAL-3-SIA: Route 10.5.0.0/16 stuck-in-active state on GigabitEthernet0/0/1.
  Resetting neighbor 192.168.12.2 (GigabitEthernet0/0/1)
%DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 192.168.12.2 (GigabitEthernet0/0/1) is down: 
  stuck in active

5. EIGRP Stub Routing & Query Boundary Containment

The most effective architectural strategy for eliminating SIA conditions and optimizing control plane scalability is EIGRP Stub Routing.

+-----------------------------------------------------------------------------------+
|                         EIGRP STUB QUERY CONTAINMENT                              |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|                               [ Hub Core Router ]                                 |
|                               /                 \                                 |
|                     DUAL Query                   DUAL Query                       |
|                    (NOT SENT!)                   (NOT SENT!)                      |
|                         x                             x                           |
|                         |                             |                           |
|               [ Branch Spoke 1 ]            [ Branch Spoke 2 ]                    |
|                 (EIGRP STUB)                  (EIGRP STUB)                        |
|                                                                                   |
|  1. Spoke routers flag themselves as 'Stub' in Hello packets.                     |
|  2. Hub router knows spokes can never act as transit routers for other sites.     |
|  3. When core routes fail, Hub NEVER sends DUAL Queries to Branch Spokes.         |
|  4. Query scope is bounded entirely within the Hub Core. 0% Risk of Branch SIA!   |
+-----------------------------------------------------------------------------------+

How Stub Routing Works

In a Hub-and-Spoke enterprise WAN topology, branch (spoke) routers have only one logical path to the rest of the enterprise (through the hub). A branch router should never act as a transit path between two hub routers.
When a branch router is configured as an EIGRP Stub, it advertises a special TLV in its Hello packets identifying itself as a stub.
The hub router records this stub flag in its neighbor table. When a route fails in the core network, the hub router will NEVER send DUAL Query packets to the stub router.
This completely bounds the query radius, dramatically lowers CPU and WAN bandwidth overhead, and makes SIA conditions impossible at branch sites.

EIGRP Stub Parameter Options

Stub Option Keyword	Advertised Routes from Stub to Upstream Hub	Use Case / Notes
`receive-only`	None (Zero routes advertised)	Pure leaf nodes. The stub receives routes from the hub but never advertises any local networks. Cannot be combined with any other keyword.
`connected`	Advertises directly connected subnets matching a `network` statement or redistributed into EIGRP.	Enabled by default when `eigrp stub` is configured.
`summary`	Advertises locally configured auto-summary or manual summary routes (`summary-address`).	Enabled by default when `eigrp stub` is configured.
`static`	Advertises static routes redistributed into EIGRP.	Used when a static default or local static route exists on the branch.
`redistributed`	Advertises routes redistributed into EIGRP from other routing protocols.	Used when branch redistributes from a local OSPF or BGP process.
`leak-map <name>`	Selectively advertises specific routes permitted by a route-map, overriding stub filtering rules.	Enables fine-grained prefix leakage without turning the branch into a transit node.

Default Stub Behavior: When the command eigrp stub (Classic Mode) or eigrp-stub (Named Mode) is issued without parameters, Cisco IOS-XE automatically defaults to connected summary.

6. EIGRP Verification & Troubleshooting CLI Reference

! 1. Verify EIGRP Neighbors and Retransmission Timers
Router# show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H   Address          Interface       Hold  Uptime    SRTT   RTO  Q   Seq
                                     (sec)           (ms)       Cnt  Num
0   192.168.12.2     Gi0/0/1           14  02:14:05    12   200  0   45
1   192.168.13.2     Gi0/0/2           12  01:45:20    18   200  0   38

! Key Output Fields:
! - SRTT (Smooth Round-Trip Time): Average time in ms to send packet and receive ACK.
! - RTO (Retransmission Timeout): Time in ms router waits before retransmitting unacknowledged packet.
! - Q Cnt (Queue Count): Number of packets queued waiting to be sent. Must consistently be 0.

! 2. Inspect the EIGRP Topology Table (Successors & Feasible Successors)
Router# show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(100)/ID(10.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply, r - SIA-Reply

P 10.5.0.0/16, 1 successors, FD is 3072
        via 192.168.12.2 (3072/1280), GigabitEthernet0/0/1     <-- SUCCESSOR (Installed in RIB)
        via 192.168.13.2 (5120/2048), GigabitEthernet0/0/2     <-- FEASIBLE SUCCESSOR (2048 < 3072)

! 3. Display ALL Links in Topology Table (Including Non-Feasible Successors)
Router# show ip eigrp topology all-links
P 10.5.0.0/16, 1 successors, FD is 3072, serno 12
        via 192.168.12.2 (3072/1280), GigabitEthernet0/0/1
        via 192.168.13.2 (5120/2048), GigabitEthernet0/0/2
        via 192.168.14.2 (6400/3500), GigabitEthernet0/0/3     <-- NON-FS (RD 3500 > FD 3072)

! 4. Check EIGRP Protocol Parameters, K-Values, and Stub Status
Router# show ip protocols
Routing Protocol is "eigrp 100"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP-IPv4 Protocol for AS(100)
    Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
    Router-ID: 10.1.1.1
    Topology : 0 (base)
      Active Timer: 3 min
      Distance: internal 90 external 170
      Maximum path: 4
      Variance: 1

Test Your Knowledge

A network engineer inspects the EIGRP topology table on Router R1 for destination prefix 172.16.50.0/24 and observes the following output: P 172.16.50.0/24, 1 successors, FD is 28160 via 10.1.12.2 (28160/15360), GigabitEthernet0/1 via 10.1.13.3 (33280/28000), GigabitEthernet0/2 via 10.1.14.4 (40960/29000), GigabitEthernet0/3 If the primary link via 10.1.12.2 fails, how will DUAL respond?

DUAL will instantly place the route in the Active state and broadcast Query packets out GigabitEthernet0/2 and GigabitEthernet0/3.

DUAL will instantly promote the path via 10.1.13.3 to Successor with zero convergence delay because its Reported Distance (28000) is strictly less than the historical Feasible Distance (28160).

DUAL will perform equal-cost load sharing between 10.1.13.3 and 10.1.14.4 because both paths reside in the topology table.

DUAL will drop all traffic because the total metric via 10.1.13.3 (33280) exceeds the original Feasible Distance (28160).

Test Your Knowledge

An enterprise is deploying high-speed 40Gbps and 100Gbps interfaces across its campus core and needs to ensure EIGRP accurately differentiates between link speeds during metric calculation. Which configuration approach must the engineering team implement?

Configure classic EIGRP with K-values set to K1=2, K2=0, K3=2, K4=0, K5=0 under the global router configuration.

Increase the EIGRP variance multiplier to 10 under classic autonomous-system mode.

Enable EIGRP fast-reroute per-link LFA under the physical interface configuration.

Migrate from Classic Mode to EIGRP Named Mode to leverage 64-bit Wide Metrics with picosecond delay scaling and throughput formulas.

Test Your Knowledge

During a network convergence event, an EIGRP router loses its primary route with no Feasible Successor and transitions the prefix to the Active state. At 90 seconds into the 180-second active timer, one downstream neighbor has not returned a DUAL Reply. What action does the router take to prevent an unnecessary neighbor reset?

The router transmits a unicast SIA-Query packet to the non-responding neighbor; if an SIA-Reply is received, the active timer resets for an additional 90 seconds.

The router immediately terminates the neighbor adjacency and flushes all routing tables to prevent a counting-to-infinity loop.

The router increases its hold timer to 240 seconds and retransmits a multicast Hello packet with an updated sequence number.

The router changes the route's Administrative Distance to 255 and injects a default route pointing to Null0.

Test Your Knowledge

A network architect is designing a hub-and-spoke WAN with 200 branch routers connecting to a redundant pair of data center hub routers. The architect must ensure branch routers never receive DUAL Query packets during core routing flaps and never act as transit points between hubs, while still advertising local LAN subnets and manual summaries. Which configuration should be deployed on the branch routers?

Configure 'eigrp stub receive-only' on the branch routers under the EIGRP address-family.

Configure 'distance eigrp 90 170' combined with IP SLA tracking on the hub routers.

Configure 'eigrp-stub connected summary' (or default 'eigrp-stub') on the branch routers under the EIGRP address-family.

Configure 'maximum-paths 1' and 'variance 1' under the topology base on all branch routers.

Up Next

9.1 OSPFv2 Architecture & Neighbor State Machine

Chapter 9: OSPFv2 & OSPFv3 Operations

CCNP Enterprise

CCNP Enterprise

8.2 Enhanced Interior Gateway Routing Protocol (EIGRP)

Key Takeaways

8.2 Enhanced Interior Gateway Routing Protocol (EIGRP)

1. DUAL Finite State Machine & Loop-Free Terminology

Core DUAL Definitions

2. EIGRP Metric Calculations: Classic Metrics vs. 64-Bit Wide Metrics

The Five K-Values and Defaults

1. Classic EIGRP Composite Metric Formula (32-Bit)

Limitations of Classic Metrics on High-Speed Interfaces

2. 64-Bit Wide Metrics (EIGRP Named Mode)

Metric Comparison: Classic vs. Wide Metrics

3. EIGRP Named Mode Configuration Architecture

Production IOS-XE Named Mode Configuration

4. Active State, Query Propagation, and Stuck-in-Active (SIA)

The Stuck-in-Active (SIA) Dilemma

Modern SIA Mitigation: SIA-Query and SIA-Reply (RFC 7868)

5. EIGRP Stub Routing & Query Boundary Containment

How Stub Routing Works

EIGRP Stub Parameter Options

6. EIGRP Verification & Troubleshooting CLI Reference

CCNP Enterprise

1Chapter 1: Enterprise Architecture & Hierarchical Design

2Chapter 2: Enterprise Quality of Service (QoS)

3Chapter 3: Software-Defined WAN (Catalyst SD-WAN)

4Chapter 4: Software-Defined Access (Catalyst SD-Access)

5Chapter 5: Device & Data Path Virtualization

6Chapter 6: Network Virtualization & Overlay Technologies

7Chapter 7: Layer 2 Infrastructure & Switching

8Chapter 8: Layer 3 Routing Foundations & EIGRP

9Chapter 9: OSPFv2 & OSPFv3 Operations

10Chapter 10: Border Gateway Protocol (BGP)

11Chapter 11: Policy-Based Routing & Route Redistribution

12Chapter 12: IP Services & High Availability

13Chapter 13: IP Multicast Operations

14Chapter 14: Network Assurance & Monitoring

15Chapter 15: Catalyst Center Assurance & Controller Operations

16Chapter 16: Enterprise Security & Access Control

17Chapter 17: Threat Defense & API Security

18Chapter 18: Network Automation, Programmability & Data Modeling

19Chapter 19: Orchestration, EEM & AI Operations

CCNP Enterprise

8.2 Enhanced Interior Gateway Routing Protocol (EIGRP)

Key Takeaways

8.2 Enhanced Interior Gateway Routing Protocol (EIGRP)

1. DUAL Finite State Machine & Loop-Free Terminology

Core DUAL Definitions

2. EIGRP Metric Calculations: Classic Metrics vs. 64-Bit Wide Metrics

The Five K-Values and Defaults

1. Classic EIGRP Composite Metric Formula (32-Bit)

Limitations of Classic Metrics on High-Speed Interfaces

2. 64-Bit Wide Metrics (EIGRP Named Mode)

Metric Comparison: Classic vs. Wide Metrics

3. EIGRP Named Mode Configuration Architecture

Production IOS-XE Named Mode Configuration

4. Active State, Query Propagation, and Stuck-in-Active (SIA)

The Stuck-in-Active (SIA) Dilemma

Modern SIA Mitigation: SIA-Query and SIA-Reply (RFC 7868)

5. EIGRP Stub Routing & Query Boundary Containment

How Stub Routing Works

EIGRP Stub Parameter Options

6. EIGRP Verification & Troubleshooting CLI Reference