EIGRP Stub Routing

🛰️ EIGRP Stub Routing: Fortifying the Network Edge and Accelerating Convergence

Category: EIGRP

Level: Intermediate–Advanced

Use Case: WAN Optimization & Query Suppression

🚀 Introduction: Why Stub Routing Matters

Enhanced Interior Gateway Routing Protocol (EIGRP) is widely respected for its fast convergence, powered by its Diffusing Update Algorithm (DUAL).

But in large-scale or hub-and-spoke networks, this speed can come at a cost — EIGRP’s query mechanism can generate excessive control-plane traffic and CPU utilization during route recalculations.

That’s where EIGRP Stub Routing becomes the hidden weapon. It:

• Restricts query scope
• Prevents unnecessary transit routing
• Enhances convergence and WAN stability

In essence, it protects your network’s core by reducing unnecessary computations from remote edge routers.

⚙️ Part 1: The Problem — “The Query Storm”

When a route disappears in a standard EIGRP topology:

1. The router detecting the loss checks for a Feasible Successor (FS).
2. If no FS exists, the route goes Active, triggering a query process.
3. The router sends QUERY packets to all EIGRP neighbors asking for an alternate route.

This process, known as Diffusing Computation, can cause major delays:

• If any neighbor doesn’t reply in time (≈90 seconds), the route becomes Stuck-In-Active (SIA).
• An SIA condition can tear down the neighbor adjacency, increasing convergence time.

📉 Real-World Impact:

In hub-and-spoke WAN designs, a query sent from the hub to multiple spoke routers causes query flooding, leading to CPU strain and potential route instability.

✅ Stub Solution:

EIGRP Stub Routing limits query propagation to prevent edge routers from participating in such diffusing computations.

🧩 Part 2: What is an EIGRP Stub Router?

An EIGRP Stub Router is a device configured to tell its neighbors “Don’t query me for alternate paths.”

When configured, the router:

• Adds a Stub flag in its Hello packets.
• Neighbors record this flag in their EIGRP neighbor table.
• Hub routers avoid sending queries to the stub when routes go active.

🔑 Benefits of Stub Routers

Benefit	Description
Faster Convergence	Eliminates unnecessary queries to edge routers, reducing SIA chances.
Prevents Transit Routing	Stub routers cannot forward traffic between two core peers — ensuring they’re not used as transit points.
Resource Optimization	Reduces CPU load and EIGRP update traffic, especially over slow WAN links (like T1).

🛠️ Part 3: EIGRP Stub Configuration and Behavior

By default, the eigrp stub command advertises only connected and summary routes

🔧 Command Syntax

Router(config-router)# eigrp stub {connected | static | redistributed | summary | receive-only}

🧠 Option Breakdown

Option	Behavior	Technical Insight
connected	Advertises directly connected routes. (Default)	Shares networks originated in the local EIGRP AS.
summary	Advertises summary routes. (Default)	Controls query scope and promotes route aggregation.
redistributed	Advertises redistributed routes from other protocols.	Marks routes as External (EIGRP Type 5).
static	Advertises static routes configured on the stub router.	Must be manually specified.
receive-only	Router only receives routes — advertises none.	Cannot be combined with other options; used for fully isolated branches.

🧩 Example (IPv4 Classic Mode)

R1(config)# router eigrp 100
R1(config-router)# eigrp stub connected summary

🧩 Example (EIGRP Named Mode)

R1(config)# router eigrp CORP
R1(config-router)# address-family ipv4 unicast autonomous-system 100
R1(config-router-af)# eigrp stub connected summary

🧩 Example (IPv6 EIGRP)

R1(config)# ipv6 router eigrp 100
R1(config-router)# eigrp stub connected summary

🔍 Verification Commands

Command	Purpose
show ip eigrp neighbors detail	Displays neighbor status and stub flag recognition.
show ip protocols	Confirms EIGRP stub configuration.
show ip eigrp topology	Verifies route advertisements and query scope.

🧭 Part 4: The EIGRP Stub Site — Advanced WAN Optimization

In real-world topologies, a stub router may still have downstream routers on its LAN side.

If configured as a regular stub, those downstream networks won’t be advertised to the core — causing reachability loss.

To solve this, Cisco introduced the EIGRP Stub Site feature (available only in EIGRP Named Mode).

⚙️ How EIGRP Stub Site Works

1. WAN Interfaces: Identified as the boundary toward the core network.
2. LAN Interfaces: Continue to exchange EIGRP routes with downstream routers.
3. Transit Prevention: Routes tagged with the stub-site identifier are not re-advertised out other WAN interfaces.

Thus, the router:

• Acts as a stub only toward WAN peers.
• Maintains normal routing with LAN peers.
• Prevents spoke-to-spoke routing through the site.

🧩 Example Configuration

router eigrp EIGRP-NAMED
 address-family ipv4 unicast autonomous-system 100
  af-interface Serial1/0
   stub-site wan-interface
  exit-af-interface
  eigrp stub-site 100:1

Explanation:

• stub-site wan-interface: Defines Serial1/0 as the WAN edge.
• eigrp stub-site 100:1: Creates a unique identifier for this site.
• Ensures no transit routing occurs between WANs.

🧱 Design Considerations

Scenario	Recommended Stub Type
Remote site with only LAN users	Standard stub (connected summary)
Remote site with redistributed static routes	Stub with static option
Remote site with local redistribution	Stub with redistributed
Spoke router with multiple downstream routers	Stub site configuration
Device for monitoring or backup (no advertisements)	receive-only stub

⚡ Key Takeaways

✅ EIGRP Stub Routing limits query scope and protects edge routers from participating in route computations. ✅ It prevents Stuck-In-Active (SIA) events by stopping unnecessary query propagation. ✅ Default stub options: Connected + Summary routes. ✅ EIGRP Stub Site adds control for dual-interface routers (WAN + LAN) without breaking reachability. ✅ Critical for hub-and-spoke WAN, DMVPN, and remote branch topologies.

🧠 Interview & Revision Quickfire

Question	Answer
What is the main purpose of EIGRP Stub Routing?	To limit query propagation and prevent SIA events.
Default stub advertisement types?	Connected and summary routes.
Command to configure stub?	eigrp stub [options]
What does “receive-only” mean?	Router does not advertise any routes, only receives.
What is EIGRP Stub Site used for?	To allow LAN EIGRP peers while maintaining WAN stub behavior.
How to verify stub status?	show ip eigrp neighbors detail
Does a stub router participate in transit routing?	No — it cannot forward traffic between EIGRP peers.

🏁 Conclusion

EIGRP Stub Routing is the shield for your network edges — it contains route queries, speeds up convergence, and stabilizes EIGRP in large or distributed environments.

By integrating Stub and Stub Site configurations intelligently, you achieve:

• 💨 Faster failover
• 🧠 Reduced query overhead
• 🧩 Scalable and stable EIGRP domains

This makes it a must-know concept for every Network Engineer, EIGRP designer, or interviewer evaluating real-world routing design knowledge.

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