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8.2 Enhanced Interior Gateway Routing Protocol (EIGRP)

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CCNP Enterprise ENCOR 350-401

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1 Chapter 1: Enterprise Architecture & Hierarchical Design

1.1 Enterprise Network Design Models & Hierarchical Campus Architecture 1.2 On-Premises, Cloud & Hybrid Infrastructure Deployment Models 1.3 Hardware Switching Architecture & Packet Forwarding

2 Chapter 2: Enterprise Quality of Service (QoS)

2.1 QoS Components: Classification, Marking & Trust Boundaries 2.2 QoS Policy: Queuing, Shaping, Policing & Interpreting MQC Output

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

3.1 Cisco Catalyst SD-WAN Architecture & Orchestration 3.2 SD-WAN Policies, Traffic Engineering & Application-Aware Routing

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

4.1 Catalyst SD-Access Fabric Architecture & Plane Separation 4.2 Cisco TrustSec, Security Group Tags (SGT), SXP & MACsec

5 Chapter 5: Device & Data Path Virtualization

5.1 Server & Network Device Virtualization 5.2 VRF-Lite & Virtual Routing Multi-Tenancy 5.3 GRE & IPsec Tunnel Virtualization Mechanics

6 Chapter 6: Network Virtualization & Overlay Technologies

6.1 LISP Architecture & EID-to-RLOC Mapping Mechanics 6.2 VXLAN & EVPN Overlay Architecture

7 Chapter 7: Layer 2 Infrastructure & Switching

7.1 VLANs, 802.1Q Trunking & VTPv3 7.2 Spanning Tree Protocol Mechanics 7.3 Advanced STP Protection Mechanisms 7.4 EtherChannel Link Aggregation

8 Chapter 8: Layer 3 Routing Foundations & EIGRP

8.1 IP Routing Foundations & Administrative Distance 8.2 Enhanced Interior Gateway Routing Protocol (EIGRP)

9 Chapter 9: OSPFv2 & OSPFv3 Operations

9.1 OSPFv2 Architecture & Neighbor State Machine 9.2 OSPF LSA Types & Multi-Area Design 9.3 OSPFv3 for IPv4 and IPv6 Address Families

10 Chapter 10: Border Gateway Protocol (BGP)

10.1 BGP Architecture, Neighbor States & Peering 10.2 BGP Path Attributes & Best Path Selection Algorithm

11 Chapter 11: Policy-Based Routing & Route Redistribution

11.1 Policy-Based Routing (PBR) & Route Maps 11.2 Route Redistribution & Loop Prevention

12 Chapter 12: IP Services & High Availability

12.1 First Hop Redundancy Protocols (HSRP, VRRP & GLBP) 12.2 Network Time Protocol (NTPv4) & Precision Time Protocol (PTP) 12.3 Network Address Translation (NAT/PAT) & VRF-Aware NAT

13 Chapter 13: IP Multicast Operations

13.1 IP Multicast Addressing, Group Management (IGMPv1/v2/v3) & IGMP Snooping 13.2 Protocol Independent Multicast (PIM-SM, PIM-SSM & Bidir-PIM) 13.3 Rendezvous Point (RP) Distribution Mechanisms

14 Chapter 14: Network Assurance & Monitoring

14.1 Diagnostic Utilities, Conditional Debugging & Path MTU Discovery 14.2 Syslog Architecture, Logging Facilities & Severity Levels 14.3 SNMPv1/v2c/v3 Security & Flexible NetFlow (FNF) Architecture 14.4 Switched Port Analyzer (SPAN, RSPAN, ERSPAN) & IP SLA Object Tracking

15 Chapter 15: Catalyst Center Assurance & Controller Operations

15.1 Cisco Catalyst Center (DNA Center) Assurance & AI Network Analytics 15.2 Cisco Catalyst Center Device Onboarding & Software Image Management (SWIM)

16 Chapter 16: Enterprise Security & Access Control

16.1 Device Hardening, AAA Architecture & RADIUS/TACACS+ 16.2 Access Control Lists (ACLs) for Infrastructure Security 16.3 Control Plane Policing (CoPP) & Control Plane Protection (CPPr) 16.4 Layer 2 Infrastructure Security Features 16.5 IEEE 802.1X Port-Based Authentication & MAB

17 Chapter 17: Threat Defense & API Security

17.1 Enterprise VPN Technologies (DMVPN Phase 1/2/3 & FlexVPN) 17.2 Next-Generation Threat Defense & Secure Access Service Edge (SASE)

18 Chapter 18: Network Automation, Programmability & Data Modeling

18.1 Python Fundamentals for Network Engineers & Cisco Libraries 18.2 YANG Data Modeling Language (RFC 6020/7950) & Model Types 18.3 REST API Architecture, HTTP Verbs, Security & Cisco Controller APIs 18.4 Model-Driven Telemetry (MDT) & NETCONF/RESTCONF Operations

19 Chapter 19: Orchestration, EEM & AI Operations

19.1 Configuration Management & Infrastructure as Code (Ansible & Terraform) 19.2 Cisco Embedded Event Manager (EEM) & Intent-Based Networking

8.2 Enhanced Interior Gateway Routing Protocol (EIGRP)

Key Takeaways Explain This Topic AI

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: R D Candidate < F D Current Successor \mathbf{RD}_{\text{Candidate}} < \mathbf{FD}_{\text{Current Successor}} RD Candidate ​ < FD 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 ( K 1 K1 K 1 through K 5 K5 K 5 ).

  • The Five K-Values and Defaults
  • K 1 K1 K 1 = Bandwidth Weight (Default: 1 )
  • K 2 K2 K 2 = Load Weight (Default: 0 )
  • K 3 K3 K 3 = Delay Weight (Default: 1 )
  • K 4 K4 K 4 = Reliability Weight (Default: 0 )
  • K 5 K5 K 5 = 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 ( K 1 = 1 , K 2 = 0 , K 3 = 1 , K 4 = 0 , K 5 = 0 K1=1, K2=0, K3=1, K4=0, K5=0 K 1 = 1 , K 2 = 0 , K 3 = 1 , K 4 = 0 , K 5 = 0 ), the full mathematical formula collapses to a simple composite of Minimum Bandwidth and Cumulative Delay : Full Formula: Metric = 256 × [ ( K 1 ⋅ BW min ⁡ + K 2 ⋅ BW min ⁡ 256 − Load + K 3 ⋅ Delay sum ) × ( K 5 Reliability + K 4 ) ] \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] Full Formula: Metric = 256 × [ ( K 1 ⋅ BW m i n ​ + 256 − Load K 2 ⋅ BW m i n ​ ​ + K 3 ⋅ Delay sum ​ ) × ( Reliability + K 4 K 5 ​ ) ] Default Classic Metric = 256 × ( 10 7 Bandwidth min ⁡ (kbps) + Cumulative Delay ( μ s) 10 ) \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) Default Classic Metric = 256 × ( Bandwidth m i n (kbps) ​ 1 0 7 ​ + 10 Cumulative Delay ( μ s) ​ )

Bandwidth Component: 10 7 Slowest Link Bandwidth in kbps \frac{10^7}{\text{Slowest Link Bandwidth in kbps}} Slowest Link Bandwidth in kbps 1 0 7 ​ . 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 , μ s / 10 \mu\text{s} / 10 μ 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 Mbps 10\text{ Mbps} 10 Mbps (Ethernet) and 100 Mbps 100\text{ Mbps} 100 Mbps (FastEthernet) were the standard. Because the bandwidth calculation is capped at 10 7 BW kbps \frac{10^7}{\text{BW}_{\text{kbps}}} BW kbps ​ 1 0 7 ​ , any interface of 10 Gbps 10\text{ Gbps} 10 Gbps ( 10 , 000 , 000 kbps 10,000,000\text{ kbps} 10 , 000 , 000 kbps ) or faster results in a bandwidth cost of 1 1 1 ( 10 7 10 7 = 1 \frac{10^7}{10^7} = 1 1 0 7 1 0 7 ​ = 1 ). Consequently, classic EIGRP cannot differentiate between a 10 Gbps 10\text{ Gbps} 10 Gbps , 40 Gbps 40\text{ Gbps} 40 Gbps , 100 Gbps 100\text{ Gbps} 100 Gbps , or 400 Gbps 400\text{ Gbps} 400 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 Terabits/sec 4.29\text{ Terabits/sec} 4.29 Terabits/sec and measure latency in picoseconds ( 10 − 12 10^{-12} 1 0 − 12 seconds) rather than tens of microseconds ( 10 − 5 10^{-5} 1 0 − 5 seconds). Throughput = 10 7 × 65536 Minimum Bandwidth (kbps) \mathbf{\text{Throughput}} = \frac{10^7 \times 65536}{\text{Minimum Bandwidth (kbps)}} Throughput = Minimum Bandwidth (kbps) 1 0 7 × 65536 ​ Latency = Cumulative Delay (picoseconds) × 65536 10 6 \mathbf{\text{Latency}} = \frac{\text{Cumulative Delay (picoseconds)} \times 65536}{10^6} Latency = 1 0 6 Cumulative Delay (picoseconds) × 65536 ​ Wide Composite Metric = ( K 1 × Throughput ) + ( K 3 × Latency ) \mathbf{\text{Wide Composite Metric}} = (K1 \times \text{Throughput}) + (K3 \times \text{Latency}) Wide Composite Metric = ( K 1 × Throughput ) + ( K 3 × 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: RIB Metric = Wide Metric 128 \text{RIB Metric} = \frac{\text{Wide Metric}}{128} RIB Metric = 128 Wide Metric ​ .

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 μ s 10\,\mu\text{s} 10 μ s ) Picoseconds ( 1 ps = 10 − 12 s 1\,\text{ps} = 10^{-12}\,\text{s} 1 ps = 1 0 − 12 s )

Bandwidth Scaling Factor 10 7 / BW kbps 10^7 / \text{BW}_{\text{kbps}} 1 0 7 / BW kbps ​ ( 10 7 × 65536 ) / BW kbps (10^7 \times 65536) / \text{BW}_{\text{kbps}} ( 1 0 7 × 65536 ) / BW kbps ​

  • Maximum Distinguishable Bandwidth 10 Gbps 10\text{ Gbps} 10 Gbps 4.29 Tbps 4.29\text{ Tbps} 4.29 Tbps
  • Supported Configuration Mode Classic Mode & Named Mode Named Mode only

K-Value Extension K 1 K1 K 1 through K 5 K5 K 5 K 1 K1 K 1 through K 6 K6 K 6 ( K 6 = Extended Attributes / Jitter / Energy K6 = \text{Extended Attributes / Jitter / Energy} K 6 = 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:

High CPU utilization on a downstream transit router preventing DUAL query processing.

Unidirectional link failure or packet drops on congested WAN links.

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